"Rewards points" and "airline miles" are forms of untaxable virtual currency and I'm astonished the IRS isn't cracking down.

Irs themselves don't put out much information on the topic, without any other guidance from them it's generally understood that financial compensation (points, miles, cash) you get from using your credit cards are not taxable because they are considered rebates on purchases. Its considered like a discount on the purchases you made with the card, kinda like a coupon.

They're a way for the already financially well-off to pay less for everything. Why would the IRS crack down on just one example of this, when there are so many others?

You're probably thinking of employer-paid business travel, passing on "air miles" to an employer. It's 2020, my friend.