Yeah, I see your point. I guess the active forces have usually had some prejudices against reserve forces -- that may be part of the issue. Also, moving experienced folks from the reserves to active shuts off some promotion opportunities for those already on active duty (and takes away strength from a reserve unit too) -- that may be part of the issue as well.
Basically, troops can move from active to reserves -- it just takes a good deal of paperwork.
As it is, most troops lose a ton of time-in-grade and time in service when they go from reserve to active. Their time gets 'compacted' and someone with 6 years of reserve time gets that converted to under a year of active time.