It seems like this is standard practice in other industries though. For example, I used to work at a pizza place. What's to stop me from taking what I learned

It seems like this is standard practice in other industries though. For example, I used to work at a pizza place. What's to stop me from taking what I learned there and starting a competitor? I also have a friend who worked at a carpenter's shop. What's to stop him from doing the same there?