Why is 5/8" called out on the ceilings?
I already have wood strips every 16" in both directions.
I was not planning on removing the ceiling, just drywalling over the
existing.

Unless there is a frequently used shower you don't "need " MR sheetrock in a
bathroom...Code may say otherwise however...The only place you need 5/8 Type
X sheetrock is the ceiling of the garage and the wall between the house and
garage...The rest can be half inch...Cheaper and MUCH easier to handle and
install...LOL...Good luck...