Probably because that was one of her lowest points, commercially. Stars was the only one to chart (for a couple weeks, peaking at #153), and none of the singles from any of the three albums charted, either. So putting those albums out on CD would be a tough sell. About the only people who would be interested would be really hardcore Cherophiles (is there a specific word for Cher fans?), and I'm not sure there's enough of those to justify a standard CD release. That said, Cher's got enough of a fanbase that a fanclub release would probably be worthwhile.
Or perhaps something from Rhino Handmade. RH specializes in limited runs (5000 or so) of CD reissues for pieces that probably would interest a select group of people. RH was the label that gave a CD release to Cher's 1969 "3614 Jackson Highway" and, it being Rhino, it was given the deluxe treatment - bonus tracks, excellent remastering, great informative booklet. The three albums you list were on Warner Brothers, who is now under the WEA umbrella. So is Rhino, which means, at least, there wouldn't be any problem with licensing issues. So who knows? It could happen.
Why not contact Rhino Handmade and suggest it?