They actually are. The Moon technically doesn’t orbit the earth, but a point near earth, that earth does as well. Its momentum that prevents the moon from crashing to earth, and it’s gravity that prevents it from flying off into space.

And another interesting fact: The moon’s momentum is slightly greater than that of the earth’s gravity, causing the moon to very very slowly move further away from Earth (About 3.7cm a year).

but it’s these momentums and gravitational forces that keep the moon orbiting (Orbital mechanics is fucking fascinating as fuck)

But what would happen if two bodies collided that are large? The force of impact would be the combined momentum of the two items as you believe. It is believed this is what actually formed our moon as early formation of the planets saw two planet sized bodies impact like you describe, the resulting force spun enough matter to form the moon (Mineral inspection of moon rock shows it contains the same isotopes as earth, which is rare if the moon formed on it’s own).