At a previous company we hired a lot of junior engineers with the idea that our relatively high pay and reasonably good work would help us build a strong generation of seniors a few years down the road. What we mostly found was that we spent a ton of money and our sr eng time training and sharpening them, only for the vast majority of them to leave after 1 to 3 years. As I've gotten more into hiring, it's not at all surprising to me now as this is just clearly in the culture.
To be clear I'm not blaming the employees, in fact I blame ridiculously stupid HR practices for the last 20 years because they built a structure in which the only way to get a raise was to leave the company, and that was also often the best way to get promoted too.