The laws are complex and varied, but there are always some things you can't consent to. For example, you can donate your organs but not sell them, you can work

The laws are complex and varied, but there are always some things you can't consent to. For example, you can donate your organs but not sell them, you can work for someone else but only so many hours, and some jobs are forbidden, etc. The idea of forbidding certain things even with consent is that pre-committing to not doing those things makes you less vulnerable to blackmail.