> why wouldn't you expect planes to be in constant utilization?

They require weekly maintenance which takes them out of service for at least 12 hours.

What we may of as 'constant utilization' is quite different in a regulated fleet environment like airlines.

maintenance would happen with the aircraft in 'wheels on ground' mode but that may not mean all systems are turned off. I expect it's like a bug in the SMC on a computer. To really turn it off you have to do some magic.

"Constant utilization" means "they aren't sitting idle", not "they aren't undergoing necessary maintenance ever".

The number of flights varies a lot by time of day, so there is nothing close to constant utilization.

There's not much reason to turn them off outside of maintenance. When they're parked, they're connected to grid power.

Airliners are regularly and routinely shut down. "Cold and dark" is a common startup procedure for the first flight of the day.

A parked Aircraft is not kept powered when there are no maintenance or other routine(cleaning/checks/certification/preparation/restocking etc.)

It is very surprising that how a lot of comments here claim the contrary.

Even when parked for next flight, until resupply and cargo routines are declared, it is also not powered.

I've flown with airlines before where there was a cascading delay due to a "plane deficit" at the terminal (not the technical term, that's my own). Not to say it's always uptime, but I imagine there are instances of constant uptime.

They can't just change things up on a dime like that. Even if it's 3 AM and most planes are sitting on the ground they can't just be used for your flight like that because they are all scheduled to take off in the morning rush a few hours later.