There are a couple things to consider here:
1. The reason Sony bought Media Molecule was because they were a relatively small (and thus cheap to buy) company with a successful new IP. Most other companies that work well with Sony (Insomniac, Naughty Dog) remain independent because they want to be that way.
2. Forcefully buying out companies isn't a good idea, especially if you're a console manufacturer. Third parties aren't going to want to develop for your system if you do that. Imagine if your games sell well on a certain system, and just because of that, the system's manufacturer decides to buy you out. Certainly would scare other companies.
3. Just because Sony bought Media Molecule doesn't mean they're in any position to buy any other companies. They may be a big company with lots of money, but they're still having quite a few financial issues (and there's always the huge debt they've incurred).
