That's too easy an answer. They get basically the same options for health insurance as other federal employees. And that insurance is not materially different from the insurance you'd get from any other major employer.
They get world-class insurance. They do pay for it, but I never said it was free.
Incentives matter. If Senators & Congress people were given only medicare and not their ACA Gold plan + perks, medicare would probably be a lot better off.
Many companies maintain VIP lists to provide better service to celebrities and politicians. When a senator calls Comcast, they are not talking to the same call center the average customer is.
Having high-quality insurance doesn't really fix the issues with US healthcare; you can still go broke with decent insurance, and the billing only gets screwier the more middlemen are involved. (Congresscritters' real insurance is that if their particular claims were denied, it would be newsworthy.) We know that our healthcare system is messed-up, but we're wary of socialized medicine because we've seen failures everywhere that Washington has been involved so far. The middle-class expects a result not like some well-run Nordic paradise, but more like what we already have, only with more forms and longer waits.