I’ve replaced all my media with 4K remuxes, when available, 1080p remuxes otherwise, unless it’s a movie I’m only getting by request, then I might get an encode. Felt good to wash away all the 1080p and especially 720p stuff. 4K with HDR makes a HUGE difference. A lot of the time the picture has been remastered (maybe the for the first time, maybe more) and is exclusive to the UHD. Also, you often get upgraded with Dolby Atmos or DTS:X tracks, which rule.
I have 50TB of capacity, 30TB used, for Movies and Shows (mostly movies). I get maximum quality 4K Blu-ray rips called Remuxes. For 4K they can be as large as 110GB each movie but never lower than 25GB.
Where I find it is private (torrent) trackers. These places are utopic is what they have available to download and organized to find everything super easy, and the trackers I’m with have a good community and we all talk and hang out in the IRC chat.
Rights holders change because companies go under, are sold, or liquidate their library (the movies and stuff they own or have rights to). The library is purchased by somebody else. Easy enough. But when producers, directors, actors, etc. and other studios who have partial ownership or distribution rights of the movie, things can get complicated during that library transfer.
Further complications arise when people die or they sell the rights to another person or company. Then you can have this knotted mess of home video and theater distribution rights (those can be separate and owned by different entities too!) which can culminate in lengthy lawsuits, or disinterest, conflict over money and percentages of profit, original contracts with the director/producer/actors/etc. getting in the way somehow, misplacing or destruction of film negatives (by far the worst case scenario), or straight up people not even knowing they own a movie’s rights.
Then there’s the stuff other commenters said. I missed some stuff I’m sure, and am not an insider. Just love movie news and movie production.