Car manuals say the same thing...it's because the manufacturers want a "window" of life expectancy from their engines.
Hell, if every single person on the planet drove their cars for 300,000 miles before they started wearing out, the manufacturers wouldn't sell as many cars! So they recommend late oil change intervals & late plug changes, so the internals & ignition systems wear out at about 100,000 miles on the nose.
Lately I've heard of manufacturers claiming you don't have to change a plug for 70,000 miles...or longer! It's a spark plug, people! I don't care how high tech that little bastard is, it's going to wear out before 70,000 miles & the rest of the ignition system is going to be required to jump this massive gap for the last 50,000 miles of that time...shit's gonna break & the unwitting owner is going to trade that thing in.
Just apply this rant to bikes...if they don't wear out, you won't want to buy a new one, will you?