Cats understand humans just fine. Don't confuse not knowing with not caring.
Last week, I was in my daughter’s apt, using the computer because mine was down, her cat was making a lot of noise, pushing a bowl across the floor. I turned and asked him what he was doing, he came over to me and started meowing, then went over and started putting his paw under the bathroom door, as if trying to open it (which he can if it is not locked). I asked him what he wanted and then it came to me, he wanted water. His water bowl was empty. That’s communicating.