In my somewhat limited experience, companies often treat 'senior devs' as team leads, building mentoring responsibilities into the role rather than treating them as 'highly skilled devs who work efficiently and do not require much oversight'.

For a mentoring structure, it is always more efficient to have more mentees than mentors, hence more junior roles than senior roles.

The 'senior' = 'superproductive' structure requires fewer employees overall, but also brings an element of risk since the work is more concentrated.

The net effect is fewer senior roles.