Pretty much every company I’ve been in or know of values a vertical trajectory instead of a horizontal one for its employees i.e becoming a manager nearly always means a faster salary progression than becoming an expert in one or multiple fields.
Why is expertise valued less?
Part of this is accountability. As you move up, you take on more responsibility for bigger and bigger things going right or wrong. This can mean a greater potential to do big things for the company, and it can also mean a higher risk of getting fired in disgrace and becoming unemployable. Most people demand more compensation to step into such a role. Agreeing to do so means you’re becoming a more deeply embedded part of the company, so the company cares more whether you’re happy, because it’s theoretically more vulnerable now if you should go rogue.
Leadership also means you’re more likely to be thinking about work (which is itself work) outside of standard office hours, which is only possible if your compensation allows you to outsource some aspects of your life, e.g. hiring a cleaner.
And if you’re in a position that’s visible within the company or beyond, they want you looking good. Nice things, clean clothes, a lifestyle for junior employees to aspire to, and an image that projects success on behalf of the company.
Also all the reasons everyone has already named. Lots of reasons. And yeah, it should be possible to move up to the same compensation level by being good at the actual work, but capitalism = scaling = constant need for leadership. Anyway, you won’t be there forever, so if you’re really that good, the company extracts more value out of you training those coming up behind you than just rewarding you for doing the same things for 30 years.