California is an at-will state and all that, but surely executives and business consultants can structure some sort of incentive package to cause workers to stay for at least some minimum duration?

If a company was willing to train me and provide me decent salary and benefits, I'd totally stay. If there was room to move up in the company, and they kept offering good salary and benefits, I'd probably never seriously consider leaving unless I hated the job or just got an amazing offer (barring events like needing to take care of parents, etc, of course). I feel like a lot of people would prefer the stability, even, especially if the company is investing in them.

The workforce leaves because the company doesn't raise the salary to account for new training.

You can't have it both ways.