Why does corki or Diana not reveal stealthed targets

He is blind

Because Diana is a moon fanatic, and Corki is flying a copter.

Makes no sense, and doesn't fit, those 2 to be able to reveal stealthed targets. Corki does give vision where his Q lands, as well as vision on all the enemies it hits. Diana's a vacuum ability that can interrupt channels, knocks up and slows. So yeah.

PeeSinger on the other hand, is blind, and slams the ground to creat vibrations he can sense, so that he can track his enemies and surroundings. Viral sonar basically, like Toph Beifong from Avatar.

revealing of stealthed targets is kind of a dumb mechanic, as it only actually matters on champions with weak stealth effects, such as , where you already know where they are, and are therefore able to un-stealth them.

against champions who have strong stealth effects like however, revealing stealth is essentially useless unless you're godlike at prediction, are very lucky, or have some very deep wards, so IMO it's probably better that there's less stealth-revealing in the game, as it's just bringing down.