Because businesses would rather have a person leave and then hire someone at 1.5x or 2x their salary than just pay the first person 2x what they were making the year before.
This happens all the time. Employers want to have 2x people at 1x prices, and that doesn't work for long.
This attitude introduces a skew where your workforce gets progressively filled with people who do not switch jobs. It's a dangerous outcome, to be managed with care. If you get stuck with those who do not switch jobs because they are too incompetent, you have a big problem. If you manage to get those who are competent but content, and not very concerned with higher earnings, you have the ideal result: stable, competent, performant team, with controlled costs.
When Starbucks entered the market here, their $5 coffee was about 3x the price of the status quo cup.
Also, if businesses can't pay the market rate for labour (by definition, the money the employee can get in an open market is the market rate) then they don't have a sustainable business model.
If Starbucks starts making coffee 3x as good will people happily start paying $12 for coffee?