You need senior engineers in order to scale training people, so it's a catch-22.
It's not cheap if they are willing to pay for it. That's why senior salaries keep going up. It's a tradeoff--they are willing to pay more for the privilege of not losing the time.
For the first N months, sure, but then, you have a senior and a trained junior, who is hopefully on their way to being a mid. So it's less of an expense and more of an investment in my eyes.
But like any upfront investment, there is a time when the costs outweigh the benefits, especially in the early days.
Too bad companies don't see it that way.
But I agree, too few companies have that perspective.