It would be short-termist if they had actual gains from their work other that salary. They don't.

They are acting rationally given companies don't seem to value long term expertise.

Now, if it was a worker- owned cooperative, that would be a different thing.

At the same time, the reason that employees leave on such a regular cadence is precisely because the companies they work for refuse to give them the salary increase that they could get by going elsewhere. Companies could solve this by giving commensurate raises.

They would rather give a 10% increase to the guy replacing you.