Rigging a commercial airline with that many ejection seats would add significant weight to the plane. You’d probably triple the cost of commercial airflight if you did this just from reduced seating capacity, and even assuming that it could be implemented without that overhead I still don’t think this would actually help much.
Imagine you’re on Ethiopian Airlines Flight 302 in a 737 Max nosediving towards the ground and the roof just opens up and launches you and all 148 of your fellow passengers out of the plane at 400MPH. Somehow I imagine that you just end up scattering the mangled corpses over a wider area.
The actual answer is that the seatbelt is there to keep your ragdoll ass from bouncing off the ceiling during heavy turbulence.