Being born into infancy is one of the worst forms of mental torture that exist in nature. You become completely helpless; not just physically, but mentally as well. The lessons you have learned in your life before, the beliefs that you have pieced together, the fun facts that you have stockpiled; all of these bits of information are shattered when they are channeled through into a new body. Many people spend the first few years of being in their new body crying because they are unable to cope with their attention constantly retrieving pieces of old experience. It is incredibly demoralizing to be reminded of thoughts that you used to be able to think of so easily. But you’ll get through it, everyone eventually does.
Being born again can be considered an absolute brainwashing, because everyone eventually succumbs to their new brain’s wiring. This includes the ability to see, the ability to wiggle individual fingers and toes, the ability to control the open or closed position of your mouth, the ability to form a thought that is understandable to your own mind - you are forced to learn how to do everything again.
However, it's not all bad, because it’s not completely from scratch. Reincarnation comes with some pre-built firmware, like facial expressions, the ability to respond to uncomfortable stimuli, and the game where people completely disappear and reappear again by hiding their face behind things. God decided that this was good enough for most infants. Any more, and the people they would grow up into might become too similar and boring.