No university class teaches modern compilers and the state of the art is in open source projects. If someone is interested in compilers, they don’t need to get a PhD in order to learn. Many major companies that maintain programming languages do not require PhDs in order to work on their compiler teams; they are willing to train you on the job. The supply of people interested is just low in general, and if you are biased towards people with PhDs and preexisting experience for junior hires, of course you will find a shortage of workers.