The problem is not even this. If you knew who has the rights, you could make an offer for selling digital licenses.

It’s often much more difficult getting to know who has what rights. There is the developer, there is one or more previous publishers (can be different per region in the world), there are investors and sponsoring publishers. And then there are sales, mergers and liquidations after bankruptcy. And no-one really knows (or wants others to know) what rights where are.