As we should all be well aware of by now, a truly original idea is hard to find in Hollywood. Even setting aside all the sequels, remakes, and re-imaginings, there's still a reliance on screenwriters to borrow story ideas from real life events. Hell, sometimes when they have trouble coming up with an original character, they just pull one out of the real world and call it a day.
And that actually makes quite a bit of sense. Most filmmakers don't live in a bubble, so when they start brainstorming ideas for their next movie, it makes sense that they'd look to the outside world for a little inspiration.
Besides, we all know people in our daily lives who are way more interesting than the typically one-dimensional archetypes that show up in most movies. Chalk it up to that whole "truth is stranger than fiction" idiom.
So not only does basing their characters on real people make the filmmaker's job ten times easier, it also helps the characters feel grounded in reality because... well... they are.