Compared to the smartphone version, the Play store in AndroidTV seems to be crippled by a lot to me.
Whats especially annoying is that I cannot manually search for software updates. This is such a basic function, but I guess we have to live without it...
I am on Sony, by the way.
The limited selection of apps is due to the developers choosing not to support the OS. Sometimes there's a good reason, sometimes they are just lazy/oblivious.
For any app that I really want, I can either (usually) find it in the Play Store or sideload it (if it's "free") from someplace like APKmirror. The later have to be manually updated, but anything I get from the Play Store can be automatically updated. You just haven't realized that yet.