Much of what you learn to get up to full speed doesn't directly transfer and is more beneficial to the employer than the employee.

>What incentive does a company have to train employees in that environment?

The normal incentive is access to a larger employee pool and cost. You have access to a much larger group of applicants if you're not looking for 100% productivity on day 1.

Also if you are willing to spend a bit of time training your employees, they are less likely to leave. Give them raises equal to what they're worth on the open market and most of them have no reason to leave.

Reducing employee churn is worth a lot of money. No employee is anywhere close to 100% effective on day 1, and from experience, training goes a long way towards improving employee retention.

Not providing training because you're afraid an employee will leave sounds a bit like a shop owner who is so scared that someone is going to steal a few pieces of candy that he locks down his store so tight no one wants to shop there. Sure a few people will take your training and run, but that's just the cost of doing business.