Why are so many of the anti tank mechanics on other tanks?

Title. Going through making item sets for champions I own, I keep seeing abilities that scale with the enemies max health on my tanks. It feels kind of weird to me to have my tanks be my anti tanks. Like, Vayne makes sense to me. A marksman with pointed anti-tank measures. But why is that on Mundo and Poppy, for two examples. I mean really this design choice is a major case of pigeon holing, which Riot claims to be against doing. Putting all the anti tank measures on tanks forces people to lane tanks against tanks. I'm not referring to those 2 champions in particular, I just keep seeing anti-tank on tanks more often that anywhere else and I feel like anti-tank should mostly be on other roles.