Why some/most videos of youtube are not avaliable in mobile devices?
I think it's because mobile devices don't support advertisements and because of that videos with ads won't play on mobiles because money is lost.
You are partially correct. Previously videos were not available on mobile because people did not gain revenue from it due to lack of advertisement. However, now advertisements do play on mobile, even the pop up ads occur, but the videos are still disabled from when ads did not play on mobile and youtubers have not updated their mobile viewing policies that they have in place.
You will find that most newer videos play on mobile, but often old ones will not/will not appear.