That already exists - there are already open, modular phone and laptop chassis standards. The market largely doesn't want that.

Standardized components tend to be significantly larger than an integrated solution can deliver. A lot of the standards also end up being pretty bad - like I'm curious, why do you point to ATX and its derivatives (ITX) as being a good thing?

Some quick examples - ATX is a relic of the days when we all had a full tower with a 5.25" floppy drive (or later optical) and maybe a 5.25" hard drive, a 3.5" floppy drive, and the GPU and CPU were an afterthought. It has no officially specified location or keepout for the CPU and memory making cooler standardization impossible. It devotes a huge amount of airspace to the CPU while leaving the GPU (with a TDP multiple times higher) with a cooler that faces the wrong direction for convection to work and an add-in-card format that makes it impossible to support the modern coolers. It has tons of power rails that are essentially vestigial because the things they used to power no longer exist (5V is only used for USB and SSDs, 3v is a leftover from the floppy drive days, etc). And all the attempts to rectify these weaknesses have been quashed and become niche unsupported standards of their own.

In the ITX space, even within homebuilder PCs, only the very smallest "standardized" SFF builds can compete with garden-variety mini-PCs, and those builds are extremely fraught with compatibility issues. A board that moves the cpu socket a half inch one way or the other might blow compatibility with popular coolers, because people have to optimize their builds to that level to make it work. And none of them reach the form factors that are possible with "slim" console-style builds utilizing a fully integrated design. Engineered solutions are simply smaller and usually cooler while doing it, because they are thoughtfully planned in a thermal and layout sense rather than having to work around layout decisions that were made literally in the 1970s by IBM.

For all these reasons, the ATX standard and its derivatives (ITX) is extremely unpopular outside the home-builder market, virtually nobody actually implements it. For example the 12VO standard is an attempt to standardize what OEMs are already doing - everyone else has already given up on ATX PSUs and gone to 12VO independently, so now there are a bunch of incompatible implementations. Things like motherboard size and screw placement vary hugely, because the market doesn't want giant full-tower cases for the computer in mom's den.

We also tried a lot of these concepts with the laptop MXM graphics card factor... but it turns out the mezzanine format adds a lot of thickness, cost, etc, for really not a whole lot of benefit. Laptops are inherently such specialized things that you end up with problems with chip placement, memory placement, cooling, power delivery, etc... and in particular it was basically never possible to swap between brands, because these factors meant the laptop just couldn't fit the GPU and actually power/cool it. It's just easier and better to design a solution that sits on the board. Socketed CPU/GPU in laptops are done, BGA is the future.

Every new standard just leads to yet another thing to be abandoned. Thin-ITX tried to fix the socket placement issue, for example. Dead. 12VO is trying to fix the power issue... dead. NVIDIA has been trying to fix the PSU power cable issue... people hated it.

It would be great if we had one standard that covered everybody's use-case, but that leads to an overcomplicated standard with a lot of nuance and boilerplate, and some things inevitably still fall through the cracks. For USB-C, that overhead is a huge amount of extra expense in the cables, devices, chargers, etc, for cases the overhead will be wasted space and weight. The market does not want to go back to phones that are as thick as chocolate-bar phones were.

Again, you can disagree all you want, but these products already exist, there is Framework aimed at the laptop market and Fairphone and others in the phone market. That is not what the market as a whole wants. But hey, government intervention can always force the issue, right?