If importance were gauged by doing dirty work without which things would fail, then the janitor is also more valuable. But janitors are cheap because they are easy to get.
If good* PMs are paid more, it is because they are harder to get. That doesn't mean they are more important, just that the company believes them to be worth it and does not believe it can get a better deal without paying unreasonably for it.
* good means whatever the person hiring thinks it means. If the hiring manager believes in auras or psychoanalysis or nepotism, then those things will determine the hire.
If good* PMs are paid more, it is because they are harder to get. That doesn't mean they are more important, just that the company believes them to be worth it and does not believe it can get a better deal without paying unreasonably for it.
* good means whatever the person hiring thinks it means. If the hiring manager believes in auras or psychoanalysis or nepotism, then those things will determine the hire.