That's why noone ever uses him. No easy projectile spam like Ness or Falco (as ineffective as their spam is, lol), and MOST of his moves have low knockback. Most. His Forward Smash definitely has KO power, and considering it's his slowest move, most people aren't expecting it. Saving it for the right moment makes it a clean KO at around 100% or so, maybe even lower damage if it's not stale. His Down Smash is in the same vote, but it's quicker, hits both sides, is REALLY spammable, and has almost the same KO power as his F-Smash.
With his quick aerials, namely his D-air, he can keep any character off stage and cheap them without needing them to be above 0% damage, even. Just be precise, counter any jump and recovery move they do with a well-timed D-air, and that's a stock instantly lost. It takes a lot of precision and practice to successfully D-air something quick like Marth's Up B, but as with anything that is incredibly good but takes massive skill to use, if you do master it, it can be a piece of the puzzle to a move being broken.
His Up B has multiple uses; it's Vertical knockback when used in midair is actually one of his best KO moves. It has the most knockback of all his aerials, comes out very fast, has a large hitbox, and when used to guard the edge, it can actually kill at 50% on a stage like Yoshi's Story or Final Destination. It's a cheap move, to say the least.
He also has plenty of other moves that have insane "priority", which means if it hits another move, priority determines which move breaks through the other, or if they cancel eachother out. His Neutral B Mach Tornado has one of the highest priorities in the game. Believe it or not, the "whorenado" has almost as much priority as a Falcon Punch. Not to mention it averages about 18%, give Meta Knight crazy good horizontal recovery (alongside his tons of jumps), and is very spammable. It breaks almost every projectile that isn't charged up to high damage, like Samus' Neutral B Charge Shot. And few moves like that are on characters that Meta doesn't utterly destroy with his speed alone.
He's not really light either, with DI (Directional Influence; using the control stick, you can slightly alter the angle a moves launches you at if you're quick. You can't be a pro without mastering it.) he can easily survive powerful KO moves at even 100% or so, if the stage isn't small. Only fresh, powerful KO moves like Snake's U-tilt or Donkey Kong's F-smash are going to off him at percentages a lighter character would normally not stand a chance at.
TL;RD
There's much more to why he's so overpowered, but to summarize what I've said, he's dangerously fast. He can get the cheapest gimp KOs in the game. He has high priority. He has ridiculous recovery (only Pit matches him, really; and Pit's Up B can be stopped with one hit and then he falls, unlike Meta). And to top it all off, he can still KO on the stage with the proper moves.