I’m using the above domain as an example because I work in it and see the calculus but there are many others. If the number of people that could do the above doubled tomorrow, they could be absorbed instantly by the demand. In a sense it is a market failure in that the demand signal is clearly there with pay to match but very few people with the necessary skills manifest.
This is true at least for finance and (technical) hedge funds.
Having observed from the sidelines over the last few years, a common pattern has emerged. Companies realise out that they have a skill or functionality gap (good), figure out what an ideal candidate would look like (fine), carve out a very specific pigeon hole in their organisation chart for that person (wait what), and then start hiring to fit that pigeon hole exactly.
I guess that makes sense if you can afford to wait for and/or poach the ideal person. For mere mortals it feels absurd.
Companies tend to be way too specific about the skills required and any learning curve is not permitted. I've used just about every desktop UI framework under the sun and a whole bunch of web ones, but companies that need react developers won't look at me for instance. There might be some highly specialized skills that can command a premium but those are mostly rare exceptions.
Along with companies not training there's also been a shift away from specialized roles and requiring all devs be experts in every tech the company uses past, present and future. When they hire someone that knows x "just in case" they're shrinking the talent pool available.