Super niche.
UCI has team cars for support.

I have had it on all my road bikes...
https://cyclingtips.com/...it-tidy-in-the-rear/

Because cyclists have jerseys with three pockets (even when racing). There's really no demand for integrated storage, since flat kits are typically removed when racing (and there is neutral support).

It's fairly easy to put an aero bento behind many road bike stems (Spech Remora below), but it won't win any fashion points with roadies. Personally, I keep my flat kit in case that fits into my seat tube bottle cage (and is thus easily moved/removed).

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Specialized has a SWAT box that mounts in the bottom bracket area of some models like the Roubaix.

Because road cycling is excessively conservative, traditional, and obsessed with imitating the appearance of pros even when the circumstances are not comparable.
Relatively basic structures based on brazed/welded round steel tubes didn't easily facilitate integrated nutrition, so pockets were used. It would be blasphemy to revisit the idea!
Some road cyclists still seem to pour scorn on saddle bags and say you should have repair kits etc in your pockets. Utter nonsense.
Sure, pros have support cars when racing and for some of their training. But they too often need to stuff their pockets from time to time, and regardless, it makes little sense for millions of people to ride bikes designed for the niche case of a few hundred or thousand elite racers. I believe road bikes should incorporate proper grown up solutions for storage and for lighting. We shouldn't be relying on 3rd party pouches attached with velcro straps. or 3rd party lights attached to handlebars and seatposts with O-rings, and other basic clamping devices. Especially when much of the focus in the industry is on aerodynamic drag reduction, it's idiotic to build "aero" bikes with no allowances for the necessities of food, repair kits, lighting, etc, the typical solutions to which completely undermine the drag reduction efforts of the design. Perhaps the central problem is an excessive obsession with fashion and convention in cycling and industry's fear of upsetting traditionalists....or their simple lack of imagination.

Havign said all that, many rides are relatively short and in predictable weather, so don't require anything more than a repair kit and a bidon. I think that does make it attractive to keep the basic frame simple, minimal, and light, but I'd like to see provision for much tidier add-on containers, behind the stem, behind the head tube, above the bottom bracket, or behind the seattube/seatpost, and again, provision for integrated lighting, which should neither look or function like an add-on.

The failure to integrate any lighting solutions on bikes outside of a tiny number of niche exceptions is indefensible. This should not be a tough problem to solve. LEDs make it very easy to integrate small lights within the frame itself, or frames, stems, seatposts, and cranks could provide simple mounting points for matching light fittings. A simple M2.5 or M3 hardpoint on the front of the headtube and back of the seatpost. Maybe the same on the chainstays for lateral visibility lights. Visibility lights should probably be standard, headlights, needing to be significantly larger and heavier perhaps make some sense as add-ons.

The 2021 Domane in the SL and SLR models has a small internal storage compartment in the down tube.

My 2017 Roubaix Pro came with the SWAT box. Works fine.

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