I read that the generator only populates fighters if there is an immediate "need" for them and will only generate new companies if there are so many fighters that the games feels like the world "needs" it. You can always force the game to generate fighters through the editor in game, if you have issues, try turning off the "needs picture" options and if you really want them to have pictures, go in and edit them to have pictures. If you want to find the generated fighters more easily you can check the box for an _ to be placed in front of their name, it makes them easy to find and go edit in the editor, but you don't need to open up an editor just to add a picture, if you do use the underscore option, you will want to edit them so the underscore is no longer present, if you are like me it is annoying to look at. lol
Edit: Either because the game feels like it needs to generate a fighter/company or because there is one already made set to make their debut.