Why doesn't anyone play Vel'koz? This dude is like the number 3 best midlaner, with a 53-54 percent winrate
winrate means literally nothing on its own
it's just a metric to measure the strength of the average player on a champion. Champion winrates fluctuate due to the meta, balance changes with runes/items, game length, etc.
For example, Singed has the 2nd highest winrate top lane in platinum+ right now, but that statistic is incredibly misleading.
Gnar and Camille are great picks into him and are strong, he falls off like a ton of bricks after the 25 minute mark, and unless one is willing to put in the time to learn a lot of champion-specific mechanics, picking literally ANYTHING else is better.
Singed having a 52% winrate doesn't mean everyone should pick him. It literally just means that the average of all Singed players' winrates is 53%. Singed has a lot of OTPs, so it makes sense. Vel is similar, his pickrate has never been crazy but he does have his one tricks.