The USPS is also, ahem, rather incompetent, and their own report [1] shows that "mitigation efforts are limited, with no substantive approach to identify counterfeit stamps in the Postal Service network. While the Postal Service and the Postal Inspection Service have increased prevention-related efforts, the quantity of counterfeit stamps in the network remains unknown."
So the increased value of the stamps, the inability to change old Forever stamps, and the lack of enforcement have created a perfect storm for the counterfeiters.
[1] https://www.uspsoig.gov/reports/audit-reports/counterfeit-st...
https://www.royalmail.com/sending/barcoded-stamps
There are still some "Forever" stamps (those with pictures on or Christmas Stamps) but the old non-QR code non-forever stamps stopped being legal a while back (but you can always swap them for the new ones for free).
I've had people try and reuse a QR-coded stamp, you just get a postcard saying there's an item of mail with invalid postage for you to collect (and pay for). You get to look at it before you decide whether to pay the fine or not and then your argument is with the sender to reimburse you or not.