Because Google frequently changes the youtube API (or just its key).
So someone would have to keep up with these changes and regularly update the client software. Apparently nobody is doing that because there's probably nobody left who wants to babysit the software just because Google likes to randomly throw a spanner in the works now and then.

I don't see this as a case of "planned obsolescence". It's just another example of Google wanting to annoy 3rd party devs away while formally adhering to the FLOSS label for PR reasons.