Lots of companies can’t afford to compete with G / Fb / etc for principal engineers. (And many companies who should hire good people cheap out and don’t spend the money.) Thus lots of important, otherwise valuable projects are staffed by people who struggle to take technical responsibility for the tasks in front of them.
This becomes self reinforcing - senior engineers have a lot of market power. They (we) generally don’t enjoy working on projects with mountains of technical debt alongside the processes which have a track record of creating them. “You’re really talented. Why would you work at a bank / corporate?”
Labor supply isn't perfectly elastic - there are only so many people who are capable of doing a particular job, and it takes time to train more.
On top of that, of the pool of folks who could get trained up, some of them wouldn't be interested in the work at any rate of pay!
By definition, the market rate should involve zero shortage. That said, search costs and regulatory barriers are significant frictions. In the midst of an unforecasted surge in aggregate demand and you have an explicable event.