why does kog have a 55% winrate

Less than a 1 % pick rate...

After the revert on his rework, people were getting used to playing him again. Some didn't know what to do with the removal of his on-hit synergy, and some just reverted to the items they bought a year before. So Riot buffed him a couple times. Now it's been long enough that people have found out how to optimize him again, and like MLD said, his play rate has dropped a ton (largely due to his lack of perceived OP-ness). Low play rate usually means only mains playing him, or only picking him in good team comps/situations, both of which have a positive influence on win rate.

If you check out champion.gg, you'll see his win rate slowly climbing over the past 5 patches; it's not just a response to Ziggs.

Because the only people who still play him are actually good at kog'maw.

Kog is crazy good with guinsoo, and considering he can lower armor/mr and does %hp magic damage, he's hard to itemize against, consider then that supports nowadays are more on the damage side and you have 2 characters doing almost true damage in tandem...