Sure, if only one airplane exists at a given time in the US. With so many flights going on simultaneously, that airplane will have to wait it’s turn to use an open airspace; hence, no simple delay.

There is a YouTuber that interviewed Air France’s dedicated ATC guy, and he even has cameras of each plane/gate to make sure everything is on time for their flight plans.

In summary, what may seem simple as holding a flight, is not that simple.