> Why should a government prohibit private parties from agreeing to anything other than those 3 things?

Because a severe power imbalance allows for abuse, and governments should prohibit such abuse.

https://en.wikipedia.org/wiki/Unconscionability

In particular, one private party has an expensive and highly educated legal team and a lot of time. The other party wanted to eat a burger and didn't have a week to do a thorough legal review of the TOS to check if they were potentially selling any kidneys for a dollar.