Are you not familiar with the Screen Actor's Guild?

Screen Actors Guild is not a traditional employment union in that your membership to SAG is not tied to your employment with given corporation. Members agree to only work with companies that have a contract with the Guild, and most members are "hired" by companies as contractors or via "loan out" companies or other such things not as w2 employees.

This makes the makeup, laws, and conditions of a SAG contract FAR FAR FAR different then what is being talked about here

The point is it's possible to have union contracts under US law where pay and title isn't based on tenure. Just because you haven't seen them doesn't mean they can't happen.

I have never seen a competent person elected as president either, I suppose it could happen but it is not likely under our current system of laws

I suspect most white collar union work is more like mine. Seniority factors in in the sense that we get annual raises, and the more years you're around, the more annual raises you have received. But e.g. applying for internal jobs, getting OT or time off (as other comments have mentioned) seniority plays no role. Further, there are pay ranges for each job title, so it's quite common that a new hire into a more senior title is getting paid more than an old timer that for whatever reason is still in a more junior title.