Fridge Logic: The Overlord's plan to take over a world bombed back to the Stone Age is to make Super Soldiers with Yor's DNA? Wha?! Doesn't he already have an army of robotic mooks with laser weapons and flying saucers under his command?
All he had was a single base on an island with limited resources and decaying technology that was centuries old. Any plan to improve his robotic troops would be considered in that case.
But they're even slower and dumber than his old robots!'
Like everything else in the movie it makes much more sense if you assume everyone is high.
Why wouldn't he want an army of Yor clones? It's Yor's world. He's the man.
If you have an Orphan's Plot Trinket as Yor did and found someone with a matching trinket, wouldn't your first thought be that this person is your long lost sibling rather than someone you're destined to mate with? It doesn't matter how hot she is!
Yor's a pseudo-caveman. I don't think he cares all that much.
To be rather blunt, Yor's pretty dim even for a caveman.