Earlier quoted context omitted.
But people are commonly lazy, and you'd find that most people who don't even consider resistance to deanonymization as a concern will just use the same name on many sites.
Speaking personally, I kind of want people to know I'm the same person in different places. I do realize this is a terrible idea from a privacy perspective, and probably a lot of other perspectives. But I think a bunch of people want to use the net this way.
My situation is a bit unusual in that I almost certainly have a globally unique name (my last name is a corruption of an uncommon name in Norway; said corruption occurred two places independently, and there are less than 500 people with that last name in Norway, and probably about the same in the US, and to date I've seen no indication of anyone combining it with my first name), my first name plus last initial is also unusual enough that I only rarely can't get it as a username.