You can also replace that with "just install Windows", or "just use macOS".

Hell, you use this for anything; "why make a new album, movie, or book when there are already thousands upon thousands of them? Yours probably isn't any better!"

If whoever is putting in the time and money to maintain those things thinks it's worth, then there must be a reason for it.

I would argue that those are either similar enough to not be wasteful or dissimilar enough to not be redundant. Lubuntu is just Ubuntu with some minor differences in default packages and configs; there's not enough difference there to be wasteful. OTOH, Ubuntu, at least for a long time, was genuinely much more friendly to beginners than Debian, partially just because they could get patches in faster, and partially because they had a looser standard around non-free packages. There was divergence but for a good reason, and the projects have largely collaborated over the years so that the source code changes are shared where sensible but they target slightly different audiences with different support systems. Well, there's also Canonical just being Canonical but there's no way to solve that.