> If I, as an employer, train you for 6 months, then you leave at month 7 for a higher salary (which you can demand cause you have more training/experience), I lose out. So I have no incentive to train you.

If you train me and I get better at my job, I deserve more money. If you don't give it to me, I will work for someone else who will pay me more.

Good companies train their employees and also do what it takes to retain them. It's a two-way street. Why should you get to extract more labor value from me without paying me more?