With doctors there are places everyone goes where you can sign up and get a doctor which is relatively easy to schedule. From there for routine stuff there's less variability, you just do your routine checkup, and you'll be referred to experts if necessary.

There's also a higher minimum standard to get an MD.

It's both way easier to find one and easier to determine they're qualified/actually know what they're talking about (though obviously there's still variance in quality just not as bad).

With therapists there's no clarity around any of this really.