It's because the software EM function in tech companies doesn't have parallels to managers in other professions.

The technical decisions are made by the high-level SWEs. The product decision and customer-facing work is done by the PMs. The EM role only exists to hire, evaluate, promote, and fire SWEs. It's very light on the "engineering" and very heavy on the "manager". It's almost an HR-type role.

In my career, all my EMs who weren't recently internally promoted couldn't read the programming language that their team writes in. Some of them have good system design skills but they eventually atrophy from disuse. It's very much a role where you hang up the cleats.

The root cause is that other professions didn't bifurcate technical leadership and people management into separate streams. The partner lawyer or civil EM is the seniormost technical person on the team. Often the software EM is the least technical person on the team.

BTW there are countries (like China) that don't follow this model. Meaning, the only way to get promoted above a mid-level SWE is to become an EM. There is no parallel IC track, i.e. no "senior staff", "principal" or "distinguished" engineers. Just young ICs and older EMs.