Yes, the GM balances - they decide what type and how hard the encounters will be. But after that decision is made, it's the job of the system to provide the GM with tools to build that encounter and help me balance things: How much skeletons provide the difficulty I want? Is a lich too much? Red dragon or white dragon?

In 5e, you don't have the proper tools imo - the challenge rating is next to useless. In PF2, you have something akin to point buy for encounters - and if it says the encounter will be "moderate threat" - then you can trust that in 99% of the cases.

But at the end of the day, as a GM, if I want to provide my players with a hard, but fair fight, I don't want to have to guess what will work and what won't. Yes, with a lot of experience I will have an idea of that, but why would I pay for a system that just offloads the hard part of their game design to me? Good encounter-building tools don't get in the way of your creativity.