Because you just have to love the Uchiha kyoudai dynamics.
Influences
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He doesn't really know how to do anything else. So he climbs to the pinnacle of talent in the art of killing, and sits alone at the top.
He has been raised for one purpose: to become the liaison between the village and the Uchiha. It is not necessary for him to be a shinobi in order to fill this position... but his talent is undeniable. To become a fine shinobi like his parents... to eventually surpass them; this is what is expected.
His father's expectations weigh heavily on him. But while he shrugs them off casually and goes about his own business, he is still influenced by the method in which his father raises him: even if he despises the things his father forces him towards.
Itachi has learned how to mold one's child to one's own liking, and force him down a single path.
Sasuke leans against his side, soft and gentle and warm, and Itachi turns his head to look at him. Deadly eyes soften, their danger hidden away beneath a layer of indolent black, and Itachi strokes the tips of his brother's bangs: fingertips measuring the worth lying beneath their touch.
He will need someone at the top with him. Life at these heights isn't living at all, unless there's a constant danger of falling off.
Fugaku raised Itachi for one purpose only. Itachi will do the same with Sasuke.