1. Learning. I find that I learn 80-90% of what I will learn at a company in the first 12-18 months. Staying at a job for too long is risky because I will miss out on new things happening in my area. In the long term this is the most important, because this is what will get me more money, and make me competitive/valuable. Example: it's hard for people in traditional data roles to do ML/DL without switching jobs.
2. I prefer dynamic equilibrium over static equilibrium. I very strongly prefer being able to go out and get a good job at any time over having a "cozy job". The only way to be good at getting a job is having practice at it.
3. Impact. I find that I deliver most of my impact on the job in the first 2 years, after that it levels off and I tend to do more maintenance stuff of all the things I've built (both systems and teams).
4. More money. This is almost a side-effect of 1-3. But also in itself: if I want to get more money in my current job, it's partially up to me, but it's also up to my manager and the company/culture. If I get unlucky with my manager/company, I'm not going to get it. Also, sometimes I hit a glass ceiling, and there's simply nowhere to go (eg. I'm already the Head/Director whatever, but the company isn't that successful and/or/but pay levels aren't that high). On the other hand, switching jobs is entirely up to me: if I am good at what I do and have a good track record to prove it, practice interviewing to be good at it, and play the numbers game (apply to lots of companies), I will succeed at finding a higher paying job.
Overall:
1. I have to be ruthless and watch out for my own interests. Nobody else will. The world doesn't owe me anything, I have to go out, work for it, make smart decisions and then pluck the rewards myself.
2. I don't think this is unfair towards companies. If a company/org is badly run, people leaving is valuable feedback. Assuming a company can hire good new people, those will deliver new impact, bring new knowledge, etc.
Having said that, there are companies where you can work and make more and more money (usually stock options play a big role here), and keep learn more and more (because the company is growing like crazy). These are the startups that are run well and are rocket ships (the FAANGs were like this).