Basically, starting around 15 years ago, there's the proliferation of bootcamps teaching fullstack development, because software startups were the new hot thing, and they desperately need generalist engineers that were capable of spinning up web app quickly. Rails was the hot thing those days because of this as well. Hence we saw many new grads or even people who change careers to fullstack development and bootcamps churning out these workers at an incredible pace (regardless of quality) but the job market took it because the job market was desperate for fullstack engineers.
During that time, the best career move you can do was to join the startups movement as fullstack engineers and get some equity as compensation. These equities, if you are lucky, can really be life changing.
Fast forward now, the low hanging CRUD apps (i.e., Facebook, Twitter, Instagram, etc) search space has been exhausted, and even new unicorns (i.e., Uber) don't make that much money, if they do for that matter. Now those companies have become big, they are the winners in this winner take all filed that is the cloud tech software. Now these companies these days have no use for fullstack engineers anymore, but more specialists that do few things albeit on a deeper level.
Today, even the startup equity math has changed a lot. Even with a good equity package, a lot of the search space has been exhausted. So being fullstack engineers these days that join startups don't pay as much anymore. Instead, a better move would be to try to get into one of these companies because their pay just dwarfed any startups or even medium / big size companies.
Just my 2c as someone who is very green (5 yrs) doing software engineering. Happy to hear criticism.