Afaik there are many therapists who are qualified and want to take public insurance (reimbursement rates are actually pretty good compared to those for MDs) but in the '90s there was a formula developed for how many therapists are needed per capita, and only that many therapists get licenses to take public insurance. Since the stigma around mental health was much higher then, demand now completely outstrips supply, and the competition for the licenses of retiring therapists is fierce.