I’ve never seen any employer offer a separate mental health insurance program.

It's just so insanely expensive there's no way for an insurer to offer it without exorbitant premiums. My brother makes $160K and spends one-fourth of that (before taxes) on therapy for himself, his wife and their two kids. $200/hour sessions once a week add up pretty damn fast. I'm a school bus driver and if I went to a therapist it would cost me literally half of my income just for myself. My employer-provided health insurance covers something like two sessions a year.