A modern, well maintained (and not thrashed) motorcycle can easily reach 100,000 miles (depending on how much or little it’s ridden).
The reason you don’t see many higher mileage bikes for sale is that they:
(a) Are largely adult toys in the U.S. and most don’t see more than a few thousand miles a year. Then the novelty wears off and they get relegated to the back of the garage where they get old and rust, their seals dry and leak and rubber perishes etc.), so they get clapped-out through neglect and age rather than high mileage.
(b) They get crashed and it doesn’t make financial sense to repair them. Even a fairly minor crash on a bike with fairings can cost upwards of $2-3K to fully repair. E.g. A replacement muffler for my Suzuki costs ~$850 just for the part).
(c) No-one wants to buy a higher mileage bike because “common knowledge” has it that motorcycles are clapped-out by 50K (they may or may not be depending on a number of factors). This being the case, the owners just hang on to them and run them into the ground (or crash them) instead of selling them for peanuts.
My current bike (bought new by me) has >30K on it and it runs and looks like new (it’s 2.5 years old).
It’s also been “crashed” and repaired twice in that time. One more crash and the insurance company will write it off (see (b) above).