Theres so many reasons for this. There's the "grass is always greener" concept that hiring from outside is better than promoting from within because the company feels like they're "winning" and "stealing talent". Not to mention the idea of paying someone more to do the job they're already doing (because they've probably already taken over the responsibilities of the promotion they want) isn't palatable

There's the fact that for a million reasons workplaces have become much more impersonal

You used to have to dial a phone or show up in person to express interest in a job. Now applicants by the thousands are available to cherry pick from digitally

The people running most of these companies didn't start them, they've probably been there less than 10 years, they aren't personally invested in the worker the way someone who turned a small business into a megacorp over a 40 year career was

The largest businesses grow by becoming more efficient, which means trying to do the same things they already do while paying less salary

Truthfully businessses have been this way for a long time, its gen x and younger that recognized loyalty isn't worth shit, your employer is counting on you to be too complacent to leave, and the culture of job hopping every few years has only really taken hold en masse in the last 15 years