The limiting factor in continuous high power engine operation is
cooling.
If you put an auto engine in PARK, depress the accellerator and run the
engine at high RPM, the heat will build up and the engine will self
destruct in (I'm guessing) less than an hour.
Take that same engine and head down the interstate with the cruise
control on and the temperature gauge will stay in the middle of the
range and the engine will run as long as the fuel lasts.

Guess what happens to your aircraft engine during ground operations?
Get it up in the air with the air flowing through the cowling and it
will be happy, too.