For the same reason we make people get driver's licenses, open water certificates, and captain's licenses. Any fool can drive, dive, and run a boat. I learned driving at 10 in my dad's lap, and drove a farm truck as soon as I could reach the pedals. Most of us did. I am not special. I got a driver's license to prove to any authority that I had taken a course of instruction, was successful at that course, and had proven to an independent evaluator that I wasn't likely to kill myself, my passengers, or random drivers on the road.
If you go on a liveaboard or other boat, the operator answers to a higher authority, namely their insurance company or maybe their certification agency. In my case, my insurance company was happy to allow me to welcome solo divers if I evaluated their skills or they presented a solo card. As a solo instructor I was considered qualified to evaluate your skills to dive solo. But I didn't want to take the time to evaluate everyone who wanted to dive solo's skills, so I just required a solo card. Due to Dunning Kruger, most who think they are awesome solo divers aren't.
OTOH, if you want to shore dive in Seattle or Rhode Island, there will be no scuba police to stop you from diving solo with any or no certification. Knock your socks off. No one will notice or care if you should or shouldn't be diving solo.