With most sufficiently large companies it all boils down to making money (which usually is why they have been able to grow so large to begin with). It is a very fine line between making as much profit as possible from users and not exploiting them so much that they leave.

Google does not have that much interest in serving new, interesting videos over what earns the most clicks and therefore advertising. A lot of users might also be pretty fine with what is already served. Although I have seen more and more alternatives popping up, several with a monthly payment plan instead. While free content is nice, a paid plan incites different business practices than one where revenue is closely coupled with the number of clicks.