Lack of enough humans to be senior devs, with many companies having tons of vacancies. They're saying that you need an influx of working juniors who become seniors, but if everyone is only hiring seniors...
That fact the senior devs can make 500k a year.
As a senior dev, I don't see the problem with that. If anything the number is a bit lower than it should be.
A tiny number of them that live in America and work for FAANG can make that. Most don't make anywhere near that.