Because it's a piece of **** to clock a car these days and it's becoming increasingly common.
There’s a guy round the corner from me who will clock anything with a digital odometer for around £20. His customers seem to generally own modern BMWs / Audis etc which have been used for long distance work trips. He regularly takes over 100k miles off, and the smarter owners will go to him every 6/12 months before the service so all the records look like they match up, with the car doing 4-5k miles per year instead of 30k. As a buyer it’d be almost impossible to spot unless you had dealer level equipment.