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?
Leadership takes effort and focus.
Having worked in orgs where everyone is expected to lead at different times, I can tell you that leading takes effort and focus - that’s effort and focus that’s not spent on your area of expertise.
Good leaders spend all their effort on making a team work better - no different than a good coach.