It might have to do with the fact that the First Class stamps exploded in price recently. It was $0.55 right before covid, but now it's $0.82. It's like printing counterfeit money, except easier. A 4x5 sheet the size of a postcard becomes worth $16. The stamps themselves have relatively little in the way of counterfeiting measures, especially if you consider that "Forever" stamps exist. So even if they introduce anti-counterfeiting measures into new stamps, a pirate could print old-style Forever stamps, circumventing the new measures.

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...