> under pay them, the worker would just get another job that pays market rate
Then the employer would go back to the H1B well, underpay another worker for 6 months until the worker bailed, go back to the well, lather, rinse, repeat.
A couple thousand dollars per H1B hire-attempt is nothing to a big business, especially if they're eventually able to pay each H1B worker a fraction of what they'd need to pay an equally qualified American worker.
Worst case scenario for the business: they lose a couple thousand dollars and a few months of employee development time, then still have the option to immediately hire the qualified American worker (or try the H1B route again).
Best case scenario for the business: they get an overqualified employee at an extreme discount, since that employee is willing to accept less just to be in America. Since that employee knows it's hard to get H1B status, they're probably willing to work for a discount for many years. (therefore the business makes its money back from other failed H1B attempts, and they retain the employee at a discount)
Six months of development is barely useful for most companies. It can take 3 months before a developer is reasonably efficient. Learning the codebase, culture, and business logic takes time in most cases.
Then the employer would go back to the H1B well, underpay another worker for 6 months until the worker bailed, go back to the well, lather, rinse, repeat.