While motocycles are high-strung, they don't have to pull around a lot of load. Couple that with the fact that there are very few places that you can actually hold the throttle open for long periods of time without wadding your bike up and it's easy to see why they last as long as they do. A modern sport bike will flip itself over before you get to full throttle and rpm from a dead stop, so unless you have a really clear road... It used to be a big deal when a 1000cc bike did wheelies on their own in first gear, now 600's do them in second and 1000's do it in third-and you better watch your ass cresting a rise in 4th on the gas-there's not much scarier than a 140-150mph tankslapper....
Then there's engineering.... And Metallurgy. Cars have Pig-Iron heads, shitty engine management, and basically haven't changed much in 50 years. Bikes on the other hand have things like linerless cylinders, titanium valves and springs, 3d ignition mapping, 4 and 5 valve heads, showerhead throttle bodies, servo controlled valves in the exhaust and airbox, shim under bucket valves, slipper clutches and on and on. A motorcycle engine is what a car company would design if they weren't trying to force you to buy a new one in 2 years. It's like comparing diamonds and gravel.
Sport Rider did a test of a 100,000 mile CBR900rr years ago. It made a few more peak hp than a new one did with a little loss in the midrange. It had gone through a few starters in it's life, that's it. Motorcyclist magazine used to do a 24 hour Endurance Racing test on 750cc bikes when they would come out, then tear them down and see if they could find anything wrong. They stopped doing it after 2 or 3 years because the GSXR's and FZR's were like brand new and the only things that got replaced during the 24 hour tests were tires.
As long as a bike looks like the owner took care of it on the outside, I wouldn't hesitate to buy as used bike. There are no Junk bikes made anymore, so as long as it wasn't crashed and repaired you should be good to go.
Dave