For the most part, artists that have placed stuff up, esp older artists have under the intent that content was being used for art/animation purposes in Studio or Poser. The gaming craze for 3d content is more or so recent with the advances of Unreal and Unity becoming more widely obtainable/cost effective by the public, to create and sell their games. The extended license is for that reason since it places the IP of the artist in a downloadable/redistributed way. Before the upswing in market for 3d game content, the content was used for 3d rendering art, and animation. So older models from older users, who may or may not be here anymore whether by choice or by fate, might not want or be able to grant usage license in this way. It sucks, sometimes, but, the best way is to try and contact the content artist, the worst that can happen is they say no. But you'll never know until you ask. If you reach the outcome to where the artist is unobtainable in contact, you can still use the stuff purchased in the way it was intended, for 3d renders and animation and have those in your game, without it violating the terms from purchase, unless otherwise stated in the user agreement. (I say that because stuff you get off some free sites, have non-commercial use clauses)
For newer models, most of us vendors are content creators, making an extended license is another product, and it sometimes isn't on our minds when making content and placing it in the store, whether to market it with an extended license. We put up our creation then start the next project. I can't speak for everyone here, just from my perspective as a vendor. Some don't want their products used for more than just the art/animation stuff because of the whole redistribution fiasco that comes with it, where models can be ripped from games, ect. It is up to the content creator, their IP. Some are ok some are not with it.