Speed is not exclusive to football. There are track sprinters, of course, but certain basketball and baseball players can put on the jets in the right circumstances. (Cut to video of 55 year old Rickey Henderson running shuttle drills in the parking lot of a minor league ballpark just because. Does this video exist? You can't prove that it doesn't.) Running is a skill that's useful over a variety of disciplines, except golf, because it'd be too fun if your opponents could run after your tee shot, intercept it, and run it back to the first hole.
The obvious difference between being fast in basketball or tennis or volleyball or soccer and being fast in football however, is this: the fast guys in all those other sports aren't trying to hit you as hard as they can. It's like if the goal of the Space Race had been to knock Laika into a decaying orbit around Venus. You're not first round material, Laika, and we have character concerns because you're a dog and dogs lick themselves in public.
Seriously, look at this:
That's Clowney running WITH you, which is not how they line you up if you're actually an opposing quarterback. Unless you're Clowney himself or someone of similar physical talent, why would you ever play football? Don't do it. Stay with us, warm and safe on the Internet. We promise we can't, and won't, catch you on foot.