Because the law requires an arbitrary limit in order to apply it to everyone equally. It’s not like on your 18th birthday you’re magically a mature adult who can be trusted to sign a mortgage. But medical care requires a much more individual approach which varies person to person. Someone who is 16 could be evaluated by a professional (who has “the ability to assess capacity to assent/consent”) to determine if they understand the situation and are of sound mind to make the decision.

This is by far a more effective way of determining one’s ability to consent. But they’re not going to have you get a professional examination every time you go to the bank for a loan. They just make sure you’re 18+ and assume you’re capable.