Because I'm a Star Trek geek, and so are the boys.
Fantasy
Sometimes, Andrew fantasizes that Xander is Captain Kirk. He so strong and manly and beautiful. In this fantasy, Xander is Kirk and he is Spock, and they share a truly epic romance.
Okay, so the real Kirk and Spock never shared any form of romance, maybe a bromance, on screen, but everyone knows the truth. And, yes, Andrew and Xander had hated each other in the beginning, not like Spock and Kirk who were best friends from day one. Maybe that would make them the Kirk and Spock in the most recent rendition of the fantastic saga.
Which, or course, would mean that Andrew would have to date Uhura, which totally blows (in a very not literal sense) and kinda put a damper on the whole fantasy. Sure, the new Uhura was attractive (according to Willow, she was 'really freaking hot'), but she was also kind of bitchy, nothing like the original.
But Andrew looks past this and continues to fantasize about him and Xander as Kirk and Spock.
Xander prefers to think of them and Sulu and Chekov.