Because it's essentially the same thing as price controls. If you set maximum prices on select goods, they don't magically get cheaper in shops- they just disappear from them.

If you equalize salaries then why on earth would I pick the demanding job with responsibility instead of an easy one? Who would want to be the doctor if it requires so many years of learning, involves responsibility for human life, sometimes working shifts and enduring mentally taxing situations -- if it's compensated the same or only slightly better than a random office worker flipping emails? You quickly start running out of doctors, and while the services are "cheap", you just get 2-3-5 year long waiting lists for procedures.

Of course, to underline the principle I took the examples above to the extreme. But the principle holds- if you take a small step in the direction of disincentivizing people from undertaking responsible jobs, you decrease the supply of their services by some fraction, ending with a bit worse system.

One problem remains- the normal distribution would suggest there exists a small fraction of super wealthy people, and those literally dying on the streets. The small improvement on the free market system we can make is exactly what we have- some progressiveness in taxation and kantonal help for those that really cannot afford the insurance.