Once a company hires and trains a junior, then they have a senior.. and they don't want to pay them a senior salary, but apparently other companies do.

The math remains simple: if you already have an employee on your payroll, how in the world are you not willing to pay them what they can get by switching at that point? That's literally just starving one's own investment.

The real issue is that the companies who were "training" the juniors were doing so only because they saw the juniors as a bargain given that they were initially willing to work for the lower wage. They just don't stay that way as they grow into the craft.