When a minute of downtime costs thousands, 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.
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.