Why don't people fall out of roller coasters?

Because of gravity. Why do we "stick" to the roller coaster cars as they go upside down in a loop? Because of gravity.

It sounds weird, but it's true. A roller coaster loop isn't actually circular. It is more of a teardrop shape that is called "clothoid," a spiral in which the radius changes constantly.

In a perfectly circular loop the radius is constant. But in a clothoid loop, the radius at the bottom is larger than the radius at the top. It's much the same shape as a standard helium balloon.

If a roller coaster loop were circular, to have enough speed to hold the cars to the track as they loop over would require 8 g's of acceleration as you go into the loop. Fighter pilots black out when they experience 7 or more g's, so this would clearly be dangerous to the human body. There have been a couple of cases of designers trying to include perfectly round loops.

One was not a coaster, but an enclosed water slide in a New Jersey park called Action Park. It was only open for about a month in 1985 and for only a few days in 1995, but both times there were enough bloody noses and back injuries that it was clear that the ride could not remain open. To avoid the problems of excessive g forces, coaster (and water slide) designers make the loops in a clothoid shape.

The rider feels the greatest speeds at the bottom of the loop, both entering and exiting. The rider's lowest speed in the loop is at the top of the loop. This way, the forces are 3 or 4 g's, which are exciting, but not dangerous.

Another way to look at it is that it is the inertia keeping you from falling out of the roller coaster. This is what presses your body to the outside of the loop as the cars go through the loop. Gravity is still pulling you down to the ground, but at the top of the clothoid loop, the acceleration pulling upward is stronger than gravity.