Another important thing is pronounceability. You may never have to enunciate your username, but people who make usernames generally make them capable of being enunciated nonetheless. This is achieved by way of constructing a username from consonant-vowel pairs, those 105 letter combinations that are phonetically viable. If we stray from pure mathematics by excluding phonetically infeasible names, we're left with a much smaller number than 324 sextillion, but still more than enough for us to never have to worry about running out of unique usernames.

Plus, the internet's so huge that having the same username as someone else doesn't really matter. Some people use names that are already associated with someone else completely intentionally, even. Say, for example, giving yourself the username of a Kingdom Hearts character.