I think part of the issue for many is a differing definition or understanding of the word friend. If someone uses it loosely and you don't, they will have more friends than you. They will not have the same kind of friends, and likely not as good of friends, from your perspective. To them, you have less friends, that's all they really see, it isn't as easy to see that someone's definition of friend is more strict or full as it is to see that it is less so.
Ugh...don't like how that came out, doesn't make as much sense as I'd like.
If you have close, meaningful friends that you trust, you will likely have less of them. Those that call mere aquaintances friends, those that don't expect as much out of their friends, and that don't feel as much of a duty to their friends, will often have more.
Quality versus quantity in some cases. If you're lacking in the friend department, unless you have "strict requirements" for someone if they are to be your friend, don't feel like you should force yourself to lower your standards. In my opinion 2 really close friends are better than 20 people you kind of know but don't necessarily trust. If you see all these people doing things that don't make sense (and I agree, they don't,) do you really want to be friends with any of them?
(Wow...that last paragraph was all I really needed in hindsight... Oh well, gonna leave it all anyways. )