He stays juuuuuuust this side of it being a foul vs. a yellow card. He also knows how to lay off just enough when he’s on a yellow and he never gets another one.
He’s an incredibly smart and self-aware player.
Sergio Ramos, for example, hasn’t learned this - he fouls a player so hard his dead grandparents feel his pain. Ramos will make sure he earns ever bit of that yellow card.
Caso is smart enough to realize that collecting yellows (and reds) isn’t a great strategy.
And yes, Ramos could learn to keep his hands to himself. I fully admit that 😉 I love El Capitan most of the time; sometimes, though, he needs to be punched in the face because he’s a moron.