Why do I almost never see a cash discount like this in practice? An I shopping in the wrong places? Or does something else prevent it?

The credit card companies hate it for the obvious reason and then the traditional reason was they would impose contractual requirements or get laws passed to prevent companies from offering a cash discount. People have posted here saying this has been reversed by federal rules, but then you're still left with two reasons.

One, inertia. Companies haven't realized they're allowed to do it now. That'll change over time.

Two, there are ways to transfer "cash" digitally without paying the credit card fees (i.e. ACH), and there are reasons to want to use digital payments -- making payments over the internet being a major one -- but ACH is ancient and it needs some kind of modern open standard in order to do things like make a payment request and determine in real-time whether the source account actually exists and has sufficient balance to make the payment. Various attempts to do that are constantly being made and constantly being fought against by Visa et al.

I've sometimes been able to get a cash discount by asking for it. This won't work with corporate places, obviously, but sole-proprietor sorts of shops - like my local car mechanic, and a print shop I used to use occasionally - have been receptive. I think they discounted by ~5% in each case. Did I suspect they're keeping the cash off-book? Of course. Did I care? Not at all. I don't expect their big-business competitors are paying tax, either.

Because that’s your subjective experience?

Canada has lots of stores that offer a discount if you pay cash. Many have a minimum purchase amount for credit cards.

In NY, I see it most frequently at gas stations.