Where else would they learn how to handle that kind of environment?

By starting at smaller airports or less busy runways, getting comfortable, then stepping up to more demanding situations.

On submarines (still flying, just underwater), we don’t throw new underway buddies into the most difficult scenarios right away. We do give them a seemingly overwhelming amount of qualifications to achieve in a very short period of time, but we don’t make them practice the hard stuff until we’re sure that they understand the fundamentals. Because much like ATC, if you make a mistake hundreds of feet underwater (or thousands of feet in the sky), you’re gonna have a bad time.

…kind of. In reality, there is always a qualified individual ready to physically stop you from doing the wrong thing, and there are multiple independent safety systems, interlocks, etc.