Pay - Almost always, wages don't keep up with your progression, and never with the market.
Progression - Not just promotions, but being given the interesting work to run with. Once someone sees you as junior, it's easier to go somewhere else to be treated with a bit more gravitas.
Morale - Quite a lot of tech leadership is terrible. Various problems make people sad.
Skill - They might actually be terrible / or skilled but terrible at actually finishing anything.
It makes a lot of sense to hop about until you find a company that constantly rewards you, but the reality is they barely exist and even in the top companies there's hopping about for the same reasons.I'm in the UK and spent many years as a contractor. It was never a case of not caring, or not wanting a long term home, and since most of the work and platforms were similar org to org, not understanding longer term consequences of decisions doesn't entirly apply.
Mostly, the negativity just writes off ambitious people. Though it might make sense given your org, like others, might just have the same reward problems.