Because that's one of two things: either 1. not feasible to actually create; or 2. way too expensive to create.
Keep in mind how much more difficult solo is to balance across 3 roles and almost 40 specs. There's a reason they restricted mythic raids to a fixed 20 people rather than the flex options from normal/heroic: it's a nightmare to balance unless you have very specific, very fixed parameters. There is just no way to do this for solo play and not have 100 people working on it all the time which shoots your production budget to the moon.
One-size-fits-all doesn't work at a high difficulty level because of the radical differences between the various setups. For normal-ish difficulty it's fine to make something that works for a dps, a healer, or a tank because it's kind of whatever, but if you want to actually make something comparable in difficulty to mythic-level content, there is just no way to have the same thing work anywhere close to the same for a dps, a healer, and a tank. It's just a structural impossibility. You'd have to change and fine-tune every little thing, and that becomes an absolute nightmare. They did something close to that for Mage Tower, and we still wound up with a limited number of scenarios that only fit kind-of-maybe for certain specs/classes with considerable difficulty differences.
And just to be clear, if the high-difficulty content like mythic raiding etc. was without power progression and was cosmetic/prestige only, it would collapse. Challenge Modes had pitiful numbers while they were cosmetic-only - the second they added something trivial like dungeon currency to the rewards, those numbers immediately shot up. That's the whole reason they redesigned CM as M+ with an actual reward track: people need that, or they won't do it.