The Hitch Hiker's Guide to the Galaxy has this to say on the subject of generation arks:
Don't bother.
The history of civilisations throughout the galaxy, attempting to escape the shackles of their local system, is littered with examples of the generation ark approach. Indeed, space itself is positively swimming with the remains of massive behemoths, which came to grief but a short time after setting out on their epic voyage of discovery.
The Universe is a dangerous place.
Quite apart from the possibility of the ark being destroyed by comets, meteorite showers, the aforementioned debris of generation ships of alien origin, bored xenomorphs who enjoy taking pot shots at anything coming within the range of their death-rays, rogue moons and bizarre but naturally occurring radiation fields against which the ark has no defence; there is also the very real possibility that the occupants of said ark will die out through disease, malfunction of life support systems, internecine war, or the mass suicide of the community following the rise of a new religious cult promising a better life in the hereafter ("It couldn't be any worse than this," their self-appointed high-priests argue) thereby leaving the ship to hurtle rudderless through the cold dark infinity of space before being utterly destroyed by comets, meteorite showers....
Nevertheless, that is all in the future for any civilisation still foolish enough to embark on such an exploit given the perilous nature of the mission. Incredibly, it is precisely this impossible dream that inevitably wins the day. Hardly a soul believes the venture to be anything other than foolish. Nonetheless, it is the unfulfilled destiny of the species to try. It is, it seems, their destiny to be foolish. They begin to look outwards.
Neighbouring planets, the argument goes, are worse than useless. Inevitably, there is always at least two or three gas giants, and those that remain are too small, too cold, too hot, not enough gravity, too much gravity, have a poisonous atmosphere... the list goes on. The only planet in the system capable of sustaining life is the very planet everybody is trying to get off. But, it is argued, there can logically only be one - or at most, two - planets that aren't too close to, or too far away from, the furnace at the heart of the system. Surely, similar conditions exist around other stars? There must be other inhabitable planets out there somewhere. Maybe.
It starts, typically, when some bright spark suggests that the local planetary system is hardly adequate for such a technically advanced society such as theirs. The fact that said bright spark happens to belong to the same political party as the unpopular, but incumbent, President with an election looming on the horizon, and the economy in a mess, has absolutely nothing to do with it. What is needed, the sparky one argues, is a Journey to the Stars! To build a ship the like of which has never been seen before! A starship! Many generations of people will be born on the ship! They will live their entire lives on the ship! They will die on the ship! Not unnaturally, this last bit tends to be glossed over somewhat.
By degrees the whole of society bends itself to the task of constructing a space ark for the most ambitious project ever undertaken in the entire history of civilisation. Forget the economy. Forget poverty. Forget everything. Nothing could be more important than 'the project' as comes to be known.
A few dissenting voices are heard from the opposition parties. "Jealousy is a terrible thing," the incumbent President is heard to say. "If they had thought of it first, they would be all for it. Why are they so negative? Why can't they just come on board?"
"Preferably, literally," the speechwriters taunt, "then we would be rid of the small-minded idiots for good. The opposition would never have the imagination to undertake such an impossible dream. Do not ask why we must go to the stars, ask only what you as an individual can do to help that dream come true," they add for good measure.
And so off they go. A selection of handpicked specimens, of the more healthy and cerebral, if slightly bonkers, sort are thrown together in a space the size of a couple of medium sized warehouses and left to get on with it.
You may think it's bad enough being stuck on the same planet for all your natural puff with no hope of escape. Now imagine what it must be like spending your entire life on a crowded ship travelling between the stars, with even less hope of escape, only to find that the planet of your final destination is already packed to the rafters with people from your home world. People who have taken advantage of a new faster-than-light means of interstellar travel invented just after your Great-great-great-great-grandparents left in the ark. A vehicle forgotten in the mists of time. A foot note in the history books. A vehicle the Home-World population couldn't even be bothered to track down, supposing they even gave it a second thought. Brave, if supremely foolish, souls cast adrift for the sake of a corrupt politician's second term of office.
When this very fate befell the unhappy Malcons of Tentus III, a small planet on the periphery of the Horsehead Nebula, war immediately broke out between the remote descendants of those who had travelled for millennia across the great void of space by conventional rocket-powered technology, and those members of the same species who happened to be taking advantage of the bank holiday weekend by visiting their holiday homes on the destination planet, having barely had time to enjoy their in-flight breakfast before having to get ready for disembarkation.
The Malcons of Tentus III are but one example, and a relatively successful one at that, of a species reaching their destination by means of a space ark. In the ensuing hostilities, however, their justifiable and incandescent rage was such that the only option available to the local authorities was to sentence the surviving Arkonauts to life imprisonment in a detention centre the size of a couple of medium sized warehouses where they were left to get on with it.
So why do they do it?
Well, apart from the politically distracting qualities of such projects, they're probably not much travelled and have yet to make contact with those civilisations that have already made the leap across the great divide. If they had, they could simply blag a ride with any of the friendlier star-hopping aliens of their acquaintance. Secondly, it is in the nature of sentient beings to explore beyond their natural boundaries, before they have fully explored within their natural boundaries. And thirdly, intelligent lifeforms are, for the most part, incredibly stupid.