Kog'Maw does not have alot more damage mid--game compared to Karthus.
http://www.leagueofgraphs.com/champions/stats/karthus
Karthus stays relevant through the course of the game and only slightly dips near the end of midgame/beginning of end game then spikes once again. Why? Because his items scales with his abilities and provides global execute. Karthus can rush into a middle of a team fight and disperse his AoE and WIN the fight all by himself, all the while his passive making him less afraid to die.
http://www.leagueofgraphs.com/champions/stats/kogmaw
Kog'Maw on the other hand struggles big time. In S5 Kog'Maw was strong because he could build Trinity Force (which was a mid game item) that used to have crit to help him get to mid-game. Triforce was really good on Kog'Maw because it gave him AS, MS, and Sheen proc that he could activate with his then manaless W. The best part of Triforce was that it used to build out of zeal, which Kog'Maw would then use in combination with Phantom Dancer into IE. The sheen proc would apply with BoRK on-hit passive and give him a massive heal. Since then Triforce has changed and isn't optimal anymore. He lost MS, sheen proc, HP, mana etc. His mid game is downright awful. By mid game typically I have Rageblade, BoRK and maybe Runaans built. His items have little to no interaction with his other abilities and relies solely on his W for his damage. He can't build CD because he needs survivability via Mallet or GA, can't build penetration since lethality AD doesn't scale with any of his abilities or his W, can't build magic pen or he'll lose out on AS.
Stating he kills bruisers/tanks in seconds is a complete fallacy. On paper his Q shred + his W % damage + BoRK + Guinsoos passive should shred tanks/bruisers however there are several factors that hold him back from doing what you stated " he often kills bruisers and tanks within a couple seconds". First, he NEEDS to have Guinsoo's at 6 stacks for the on-hit to proc and the stacks wear off after 6 seconds of not AA which is a really short time seeing as how Kog'Maw W lasts 8 seconds. Kog'Maw's damage is at max efficiency only when he is applying the phantom procs to create that psuedo shred appearance on lifebars. Secondly, his Q must land on his opponent for the shred to apply. The skill shot is clunky and unreliably unless its at point black range. The % shred exhibits diminishing returns at higher levels of resistances and is out shined by LDR or Void staff. Sure the ability provides "free" resistance shred only if you land it and apply damage unto it in 4 seconds. 4 seconds isn't a long time unless they're locked down. His W % damage is great, no question about it. However it's easy to avoid by simply baiting it out for it to expire then jump on him, or jump on him before he can activate it. Additionally, his W damage doesn't scale with anything but his attack speed and enemy health (and 1% ap ratio). So when he reaches late game with max AS and max HP on enemy level the scaling stops. Arguably, his damage late game is top tier but crit builds on Caitlyn or Twitch prove to be substantially better than flat on-hit damage Kog'Maw builds.
This is why no one plays him.
His DPS is amazing, but it really has to have strong counter-play to be healthy for the game.
Strong counter play?
Like having one of the lowest MS in the game and building little to no mobility all game.
Maybe having one of the lowest base HP in the game, completely disregarding he has no means of escape via dash or blink or MS boost.
Or having no source of consistent reliable mobility/anti mobility (ie Ashe arrow for anti mobility, Ezreal E for mobility)
Must be paired with Lulu to compete vs most other flexible botlane combos
W on 9 second CD with no source of resets.
Other 3 abilites output pathetic damage for their mana costs.
Entire team must protect Kog'Maw in order for Kog'Maw to succeed, in comparison to Caitlyn 2v4 with a wombo combo support (leona, morg)
Must have 6 items to "shred" entire teams.
Awful early game and weird AA animation.
R is a flaccid execute and a mana expensive poke for the small amnt of damage it does.
I could go on and on dude. Also, I looked at your match history via several websites and I dont' see any games where you've played Kog'Maw. If it was some time in the past, then I'd like to inform you that times and meta have changed and Old Kog'Maw is drastically different than current Kog'Maw.