> faster boarding as people aren't trying to cram their bags

Checking and loading baggage takes way longer, not to mention the unloading and waiting at the other end.

but it happens at a point where time is less critical. After a plane lands and the passengers depart, there is a race to clean, restock, and and board customers for the next flight, so the plane can get back in the air. Any part of that process that can be made more efficient benefits airlines. Checking luggage likely reduces complexity during that narrow window, and adds it before and after the flight

Not with containerised baggage