Are you serious!? Here I always thought to myself there was no real reason to use anything beyond CRC32 because the chances of an actual (as in not intentional) collision are practically 0 and I didn't think anyone would be enough of a jack ass to do it on purpose (silly me :|).
I can elaborate on this a little bit. There are "dumping groups" that take an inordinate amount of pride in what they do, so they feel it is necessary to credit themselves in the game with an intro or something. Then to spite archival projects like no-intro, they can hack the modified CRC32 to match the unmodified version's. CRC32 + SHA1 would make this nearimpossible because you simply cannot fake both checksums at the same time.
So yeah, for a while people were just like you, considering only deceit and vandalism out of boredom or twisted self-gratification. But then this less apparent motive became clear, and it wasn't worth risking the verification status of everything when it was so easy to just use two and be done with it.