LFR is fine. I don't like it because it's too easy, and I dislike Normal raiding because having to go through the hassle of manual PUG interfaces (Just like M+) is annoying - and joining a raid guild and adhering to a schedule for a decent challenge is also a lot to ask. I think a better question is why we don't have some system in place to allow Normal / Lower M+ automation like we do with RDF and LFR. Normal and lower M+ levels aren't exactly mentally challenging - and FF14 has shown that it works (At least on the raid-equivalent queues).
The issue isn't the length of a raid, it's the commitment and schedules you need to put aside. Not everyone has the drive to play regularly enough to adhere to one, while others have sporadic schedules which demolish any ability to consistently raid. I'd play a shit ton of Normal raids, M+s and really long dungeons (Like BRD) / raids if it was convenient to just match up and do it.