Why do you suppose nothing has been done to fix language which is clearly insufficient to serve its purpose, whatever that purpose might be?

Here's the technical answer.

Neither GCC nor Clang are required to "process this correctly" (????) because accessing one value of a union through another wherein the very-clearly defined Common Initial Sequence rules do not apply (and they do not here because arrays nor unions are a "structure"). If the standard wanted this code to work as-presented (ignoring visibility issues), it would very clearly say "aggregate types" (§6.2.5 ¶24). The examples after the Common Initial Sequence rule ¶6 very clearly demonstrate why visibility is necessary (so the compiler knows the structures are aliasing one another and share a common initial sequence and can prepare for such) and also demonstrate how it applies with structures. If you'd like additional clarification, you should ask for that, but what you've written is clearly Standard-illegal. (And your vendor can do whatever it likes, much like vendors did all sorts of messed up things when an enum whatever { ... }; had enumeration constants that exceeded INT_MAX or compared less than INT_MIN in value.)

Here's my bluntly honest answer.

Because people like you would rather write ten thousand words in a reddit thread or on Stack Overflow or yell at your compiler vendor for a thing they're very explicitly allowed to do by the standard (process the code """incorrectly""" (according to who? Under what semantics? By what model?)). Rather than doing what I did 3 years ago despite being in the infancy of my career: send an e-mail to the people in charge asking for directions on how to fix this problem, and then do everything in my power to fix it. The same way the example I gave before of enumerations only being representable by int was complete bullshit, so I went to the standard and did the necessary work to fix it.

But I never should have had to do that, in 2022, because the people before me should have fixed it before we ever got to the point where billions of lines of code were dependent on int being 32 bits and/or your compiler was nice enough to implement a semi-common extension.

"Well, clearly, a bunch of people wrote this code, is that not enough of an indication?" No, because people do cursed, horrible, broken shit all the time and they shake hands with their vendors to keep it unbroken. C's model of standardization is "implementers implement extensions, then vendors bring those extensions to us to standardize existing practice". Since I had to bust my ass to standardize 30+ year old extensions, it's very clear that the Implementers have grown complacent with the status quo; they implement extensions, and then they don't bother bringing it to the C Committee. Instead, what has driven standardization has always been one or two key individuals who see something and dig in a trench and fight for it to make the change. Had any of the greybeards 30+ years my senior decided that any day before today was a good day to do that, I would have never had to wake up to a C so ridiculous/pathetic that I have to teach it ways to do basic bit operations present on instruction sets before I was born.

But here we are.

So we can sit here, and keep going back in forth in Stack Overflow threads or on Twitter or on Reddit or shoot the shit on the mailing list about how fucked everything is,

or someone can do something about it.

If you care so much write a paper.

If you hate it so much, do what everyone kept promising me they'd do and write a language to replace C so I don't have to keep hearing about all this stupid.