Medicare's mental health benefits are lower than Medicare's other benefits. Outpatient mental health visits are only covered up to 50% of very low "reasonable and customary fees", and that plus the substantial overhead of billing Medicare is why many psychiatrists who provide outpatient mental health treatment choose not to accept patients on Medicare.
There are three possible relationships that physicians can have to Medicare.
For the first two options, the government dictates the fees doctors may charge. I have chosen to opt out. This means I can set my own fees, but Medicare will not pay you anything if you see me. However, some supplemental insurance policies will.