It's because in these environments where corporate cancer has metastasised, programmers are not in charge of hiring programmers, or much of anything when it comes to decision-making really. HR is composed of people who are not programmers. They are looking to hire people with a list of shiny hot new web stack keywords on a resume, because they have literally no other concept of how to filter candidate applications. So they end up with a bunch of hot React devs and nobody capable of making software that is fit for task.
Also, let's say team A (10 C app/systems programmers) in a company asks HR to look for a C developer and HR comes back to them with 10 great web developers to be grilled by the engineers of team A - what happens then? Does team A shrug and say "welcome to our C codebase, we shall now rewrite it in tailwind or whatever because you are now here!" - I really don't see how it can play out