I think a large part of the senior interviews being difficult is also how difficult to determine if someone is truly senior or just has X number of years of experience.

I've been part of many interviews and talked to even more senior devs that wouldn't pass as a senior dev where I work or for any of the clients I work with.

There seems to be a lot of worked 10+ years in the field, but still doing the exact same thing as the first year and haven't truly advanced to being able to lead a project and development decisions. Unfortunately many of these "senior" devs knew less about design patterns, architecture, code reviews, security, etc. than the juniors and mids we were interviewing. The interview process we follow is pretty similar for all levels as we determine where to place someone after the interview.