Airlines have to pay the airport ground crew to load and unload checked luggage. It doesn't cost the airlines much extra to have really slow boarding and unboarding, nor does having the cabin packet to the max with carry-ons affect them much.

it can cause take-of delays, which can easily cascade and affect many flights. I presume these can easily cost cash, no?

It can in extreme cases, but based on totally unscientific observations, the number of flights delayed significantly because people spend too long time stowing their carry on is tiny. Even just looking a boarding related delays, I'd say people simply not showing up at the gate on time is a much bigger problem.

They just close the doors.

Even a small airplane like a 737 costs millions of dollars to purchase and (off the top of my head) another million a year to maintain. When it isn't in the air, moving people who paid for tickets it is not earning its keep.

Sure, but turn-around times are affected by many factors, and the time it takes people to stow their luggage is probably one of the least significant.

Or, not cram so many people into the space originally designed for half as many. Then everyone's roller bag would fit just fine.