Cuz they are awesome. Especially if you're buying vintage parts, they pretty much last forever, near as I can tell. Mainly because they are simple, but also because they have very little plastic except for some of the bushings. Makes you feel connected to the bike, makes you appreciate the fine derailleurs you have on there. In the age of steel everything felt expensive. In the age of plastic, nothing does. Plus you never have to tune your derailleur. Gotta tune your thumb but you get used to it.
Swap the left and right and mount em upside down, you might like em