Because there is nothing about a high-deductible plan that insures better against catastrophic outcomes.

I do have an HDHP+HSA plan, but you still have to run all your routine care through the insurance, (a) so they negotiate a lower price and (b) so it is applied to your deductible.

Truly "paying cash" for routine care would be difficult as most doctor's offices have no idea how to quote a cash price for services, unless you go to one of the very few cash-based practices.