This exact question was asked here by another member, not that long ago... funny... x_x
The reason, of course, is because of what the other person said, though... as you may have noticed, e-mail services automatically do the same, by suggesting the same name with numbers, when you try to register and the name you want isn't available. I personally have never registered, anywhere, with random numbers, though... I, too, find it to be sort of annoying... or at the very least if I'd be the one using the name with the random numbers.