This is a complexity issue. You can fashion a ventilator out of basically old vacuum cleaner parts. N95 masks are spun with what amounts to an extremely fancy cotton candy machine that spins the fibers together. The machinery to make this is extremely complicated and takes 6 months or more to build, by which time there may be no demand. No amount of invoking the defense production act would change this reality.
https://www.npr.org/sections/goatsandsoda/2020/03/16/8149292...