[roncesvalles]

I think the reason is purely monetary.

Software developer compensation spans a very wide range, especially in the USA. Devs with the same years of experience could be earning anywhere from $50k on the low end to $500k on the high end in the same city. The "upward mobility" across the range is relatively easy and not hindered by your credentials (i.e. you aren't permanently barred from the highest end jobs because you didn't do your undergrad at Harvard, Yale or Princeton).

The fact that the USA allows immigration means that this compensation range applies globally. A dev in Tallinn earning €80k can aspire for the $500k role in San Francisco and actually have a decent shot of getting it.

It's almost unreasonable to expect employees to remain loyal in this situation.

The phenomenon where people stayed in the same job forever happened because they couldn't really go anywhere else to make more. Indeed, at high-paying tech companies, you will find many devs who have been there for decades.