Seatbelts are optimized for easy release. They have a big red button and using it is muscle memory for the vast majority of people who ride in cars. This is good because there are a lot of conceivable situations in which one might need to exit a car in a hurry. It's possible that a seatbelt or its release might cause problems in some situations, but they're designed to make this as unlikely as possible.

Strapping people into an aircraft such that they effectively cannot release themselves in any emergency is completely insane, and in this case it's a decision that led to 5 deaths. If they can't design secure harnesses that can also be easily released, or can't trust customers with such a responsibility, then they shouldn't be running open-door flights at all.