Comment by Tharre

> Seniors come from juniors. If you want seniors, you must let the juniors write the code.

Companies know this as well, but this is a prisoner dilemma type situation for them. A company can skip out on juniors, and instead offer to pay seniors a bit better to poach them from other companies, saving money. If everyone starts doing this, everyone obviously loses - there just won't be enough new seniors to satisfy demand. Avoiding this requires that most companies play by the rules so to say, not something that's easily achieved.

And the higher the cost of training juniors relative to their economic output, the greater the incentive to break the rules becomes.

One alternative might just be more strict non-competes and the like, to make it harder for employees to switch companies in the first place. But this is legally challenging and obviously not a great thing for employees in general.