My 19 year old summed it up perfectly. “Dad, why can I go halfway around the world and die for my country, but I cannot sit in the living room with you and have a beer?”
Because of insurance companies.
Look at most of the regulations and you will find insurance companies benefitting.
When I turned 18 I could legally buy and consume beer in South Carolina but I could NOT vote until I reached 21. Today the situation is reversed. Personally I think 18 year old citizens can more safely be trusted with a beer than with a vote.