I think they're not seeing it because the value you've written about above is from your perspective as an end user.
If there was enough value in it from their perspective as manufacturers, I think they'd be all over it.
From my perspective as an end-user, I have no need for a combo.
My TT-800 weighs just a smidge over 7 pounds and the cabs are 43 and 38. So around 88 pounds total, but obviously modular.
The last combo I gigged with weighed 80 pounds (SWR Redhead - original).
I think that coming up with combos that enough people are going to prefer over today's lightweight amps and cabs is a big enough ask under any circumstances, let alone keeping them at any kind of competitive price point.
My comment re: their perceived value was in response to @gtirard's suggestion that they don't see value in it because the market is too small. In truth I have no idea why they don't make more combos but lack of demand is a decent guess. I suspect that Berg/MB aren't interested in 'price point' offerings to cater to those on smaller budgets or students. Still it's interesting to speculate about why they might not see value in it from their perspective.
To be clear, for me personally the value in a combo from either company would be - meet or exceed the performance I can get out of my D-800/REF210 rig in a smaller package that's lighter, price isn't really a concern.
But I'm sure your last comment re: the big ask in competing with modular setups is probably right.
I well remember the SWR Redhead, I used to see them on gigs where backline was provided. Didn't they all come on wheels? 88 pounds? Yep, those days are gone for me.