Its because ICQ didn't *used* to be owned by AOL and back then it probably seemed a lot more sensible to use a number to identify someone uniquely rather than an arbitary name which world+dog had already registered leaving newcomers with even harder names to remember like bob213213.
I believe it's so everyone can have the Username of their choice as the number is unique...
Doesn't it really mean that no one has the username of their choice, as everyone has an arbitrarily-assigned number? :)
Remember that Compuserve used to use only numbers instead of usernames. Then at one point they began to let existing members choose a custom username, letting most "senior" members choose first before letting new signups begin to have custom names.
I might be wrong, but I'd expect that most users would prefer to be able to choose a username, even though it can be frustrating trying to find one that isn't taken.