to some of us it's comedy of the commons

Executives want to think of their employees as replaceable cogs that can be abused and replaced to turn a profit. Training is a cost to them and of course your whole logic holds. It is really a contradiction of capitalism. Tragedy of the commons is a capitalist apologetic invention, believe it or not, to justify privatization. Capital needs to maximize profit, it will do so best with a skilled workforce, but it is against the interests of individual capitalists to train workers that they believe will just leave after being trained.

Extending the contradiction, these employers are the same ones that avoid promoting from within or giving raises to retain workers. They think they're maximizing profits by avoiding raises for 2-5 years but then eat the larger costs of constantly interviewing candidates and paying them even more than those who left. And don't even think about the dead weight of the bloated management structure, another contradiction intended to discipline labor and validate the capitalists.