A bike without an odometer is "true mileage unknown" (TMU) which makes selling/insuring/valuing it somewhat difficult. On a custom chopper that is normal. Many choppers don't even have speedometers. But on a normal bike missing such instruments is seen as a red flag that the bike is stolen and/or made from stolen parts.
odometer value is easily modified, even with digital ones. Even then, it is often not accurate at all, especially on motorcycles, due to the fact that it is usually driven by a cable attached to the front wheel (just disconnect the cable and number never goes up).
Maybe in the 1980s. On a modern bike it is integrated into the transmission, usually on the drive sprocket. And then the signal is used in slightly different ways, with the odometer being accurate and the speedometer generally reading a few % higher than the actual speed. Some bikes may also use it for electronic engine/emission controls too.
Not all bikes. 2023 DRZ 400sm still has a cable (they haven't modified the design in years). Most scooters/motorbikes are cables too.
Even still, it is easily hacked on almost any vehicle (Tesla is a great example of a connected car making it much harder) and there is plenty of YT all about it...
The same reason your cell phone fell out of your pocket in the driveway before the trip. So that it never happened.
I think this is easier with bikes because there's more repair culture and more off-roading. Off-road bikes generally have all non-essentials stripped and left in a box, blinkers, odometer, fancy fenders...