If you saw the entire experience from the view of LFR, then nobody would raid anything higher.
LFR is a small portion of the experience, a small portion of the content, not "the content" but "some content".
It is a different experience, and is different content in every difficulty.
It might not be a great way to spend that time, but the community, your fellow players make LFR a necessity, and ensure that people are stuck there.
Flex does nothing to solve the core problem which people keep refusing to talk about, the organisational requirement where someone can make the rules.
LFD offers exactly the same feature but on a smaller scale, yet I see little complaints in comparison.
You raid normal or flex because it is simply not the same as LFR.
Therefore LFR is not seeing all it has to offer, where only the highest difficulty does that.