A couple things:

If WD were to start buying sandisk share-by-share, then the price would go up. By the time they owned 50+% their average share price would be a lot more than $86.

Owning 100% of sandisk is more than 100 times as valuable as owning 1%. With 100%, WD can tell sandisk what to do, has access to their IP, can merge steps in the production process, etc.