Because employers have incentivized this behavior by not paying employees what employees know they'll get when they've attained the skills and experience they need.

An alternative route would be to just increase the compensation of juniors to make sure that trying to switch is a less lucrative option, though.

I keep being told that the only way I'll be able to get a decent pay rise is by jumping ship once I'm more employable. Maybe people won't do that if the companies don't incentivise it

This is sort of like trying to wipe out all the smaller-than-3cm fry, so that you can succeed in catching only the biggest fish in your net. It's so illogical and maliciously counterproductive I fear you may be correct.

In this field, “senior” is a state of mind, there is no point in chasing promotions + compensation increases in your 1 company

I have a lot of experience and job hop too, I just never put the 3 month stints on my resume and keep the 2+ year engagements on it

Its an adaptation. I don’t think avoiding juniors helps you