Maintenance much more frequent and more expensive.
You can get a set of 4 car tires for, say, $500 that will last for, say, 50,000 miles. You can have 2 motorcycle tires for, say, $500 that will last 10,000 miles.
Rental cars are plentiful and used by the multitudes. Fleets bought cheap and sold probably for almost what they paid for them.
No where near as many rental places for bikes. Market rates.
Etc, etc.
Yes, I agree, it's a damn shame! I really want to ride Europe. I could ship my bike over, not! I could buy a bike there then sell it, not! Or I can just pay what it requires to ride one of their bikes. OR, not do it!!