Given class variance, I'd imagine it is harder to design content made to be balanced for an unpredictable composition than a fairly predictable composition. Raids are fairly predictable in terms of comp. Dungeons are not. Dungeons also need to be susceptible to affixes, which adds a whole layer of complexity, and given the timed aspect of M+ you have another layer that has to strongly counter exploits that could skip time. It's a mess of factors that looks like a nightmare to develop and balance for.
The bottom line is that what you are asking for is an absolute fantasy and is clearly never going to happen.