why some themes get in wordpress list and thounsands of them not

I have found some themes that might be good for what i want but they are not listed in official list (you cannot find them using the add new theme search option in wordpress admin)

But i ask myself if the fact that they are not included and that they are only available in the author web does mean only that author didnt take the time to upload theme to wordpress official repository

or if it is because only themes passing some tests are allowed to be in the repository

and hence that those not official themes could hide obscure sponsor links, cheats to steal wordpress admin login info, hacks that would not work well with wordpress core code, or whatever things that you should not want if you know they are there.

in short.

would you use a theme that is not in official repository.