I can sort of understand how others slash them, but I personally can't see it.

Me neither. As far as I can tell, Xander would probably vehemently insist on being straight if he ever crossed paths with the idea of being with another man. And even if Spike did have that one time with Angel, he clearly prefers the women too.

My understanding of the prevalence of Xander slash fics is that he's the character male audience members are most likely to identify with, so for guys who are into, say, Spike, then shipping them is a way to play out that fantasy. Then again, this is just speculation since I rarely ship slash pairings.