This is normal. Senior engineers are productive fairly quickly and can work independently without much or any oversight. Contrast that with junior engineers. They take longer to become productive and even worse require a lot of oversight and training/mentoring from your senior employees meaning that you're dragging down the productivity of your existing employees. Typically you need several senior hires per junior employee. Long-term you should have both, which is why bigger companies invest in training programs, where they hire new grads/bootcamp grads at specific time frames. As an industry we do need to work on how to make junior engineers effective faster.

I think it's mostly a "no bootcamp grads" filter.

As time goes on and the industry matures, senior becomes an easier bar to clear (more people with a few years of experience) and those people tend to be safer hires.

Also, some companies slap on a senior title to attract more candidates (this results in title inflation over time).

Honestly, the terminology is ridiculous and exhausting. There's no real definition for senior/staff/etc responsibilities and qualifications.