There are various different explanations across universes. But the biggest issue is - why can't vampires just drink animal blood? It has to either be some kind of magic, allergy or inefficiency usually. Regis exists to show the book's theme - that not every monster is a monster, and that not every human is human. So he needs a reason not to hurt people, otherwise he's also a monster.
In BTVS they can. There's a vampire who's cursed with having a soul and is therefore unwilling to hurt humans, so he just toddles down to the butcher and gets blood from them.
Soulless vampires either like the hunt or just prefer the taste of human blood. For instance, some vampires just raid blood banks instead of hunting humans.
I like vampires in the Pratchett universe, who often see human hunting as a sport and leave religious symbols, stakes, and garlic around the castle to level the playing field. (Not to mention the black ribbon society, the vampire equivalent to AA.)
Germany reporting in: There are bushes in nearly every park in nearly every city over here where men are waiting to suck off other guys. No joke. That's the reality.
Yeah, but what quality blood are you getting? Vampires hunt. They select. "hey come lookitthis afore I flush it!" is going to get you some dudes, but they're gonna taste bad.