Looking back we see clear signs of a seller's market in talent: companies went out of their way to hire anyone remotely competent. Once hired, devs could hopefully rely on their company's mentorship, training, and experience opportunities to raise themselves up, and companies could rely on the same for retention. Coding boot camps sprung up to give people just enough skills and credibility to get their foot in the door. Salaries were high.
That gold rush has since dried up. Early stage funding has plummeted. Fewer and fewer companies want to go public. Large companies like Google, Facebook, etc. now dominate their respective product categories. Why is it now difficult to find senior people? Because for someone with years of experience and/or formal training, these companies offer the holy trinity of excellent pay, top-tier perks, and opportunities for career growth.
My personal opinion is that many of these junior devs simply aren't very good. As someone who conducts interviews at one of those aforementioned big companies, I'm consistently disappointed by how poorly most candidates perform on basic interview questions. And I don't mean the kind that test book knowledge of some obscure algorithm, I'm talking basic coding tasks.
From where I stand it's a matter of an oversupply of inexperienced, under-qualified developers that are no longer being absorbed into startups desperate for warm bodies to do rudimentary coding tasks, coupled with a strong demand for senior folks from big companies with the coffers to pay them.