I send you a letter with a fake stamp, you pay.
The £5 is an inconvenience, something you’d pay while tutting “what is the world coming to”.
The sender is none the wiser.
7.3bn letters delivered in 22-23 (https://www.ofcom.org.uk/__data/assets/pdf_file/0032/272795/...)
If 0.01% of that are tagged as counterfeit, that’s £3,650,000 in fines.