That’s a great detail! I’m going to expand on it in case OP reads here

Gunpowder deflagrates instead of detonating. It burns instead of booms. Well, it does boom from our perspective, but if you watch in slow motion it takes a lot more time than dynamite. This gradually increases the gas pressure to start the bullet moving.

Bullets fit very tightly with the barrel. Imagine a tennis ball stuck tightly into the end of a pipe. You could punch it with all your strength, but it wouldn’t go very far and you’d probably damage yourself, the pipe and the ball. If you start pushing lightly and get it moving and then push harder, you’ll expend the same amount of energy and get it much farther down the pipe without damaging anything.

This is also why some calibers benefit from a longer barrel. If the bullet leaves the barrel before all the powder finishes burning, it will miss out on some velocity. Some calibers are designed to burn quickly so barrels can be short, but they also need to be designed strong to handle the higher peak pressure.

On top of all that, if the gun uses some of that recoil energy to do something useful like load another round, the impulse will be even more spread out on the shooter.