Why is it considered so sexually arousing just to see a guy take off his socks for some people?

Who knows? And what would change if you did know?

One of the amazing things about humans is our ability to develop fetishes -- sexual responses to things most folks don't consider sexual. For example, some people are excited by red vinyl.

This is almost certainly part of nature's attempt to make sure as much sex happens as possible. Two people who might not otherwise be attracted to each other might have intercourse just because one of them wears red vinyl (or takes his socks off). The attraction may be rooted in a fetish for socks, ankles, the way the foot moves when the socks are coming off, or the foot itself. Remember, a lot of folks enjoy watching people strip! So enjoy.