You can do work on the side without disclosing your employers trade secrets, I would hope. There is nothing mysterious about yet another CRUD web application. Besides, confidentiality is covered separately and an NDA can stand on its own.

I suspect employers love restricting your right to have a side hustle so that they absolutely totally own you. If you have no other means of income besides your job they can lean on you pretty hard and there isn't much you can do about it if you like having a roof over your head.

You're presumably already bound to confidentiality.

If someone wants to trade their employer's secrets, there are MUCH easier ways to do so than creating their own side business. This looks like a straw-man argument.

This is not about trade secrets.

Are those contract clauses limited to trade secrets?

They often are not limited. In essence the company claims it owns all your creative output while you are employed.