Why are there cat's with no fur?

Why are there humans with no hair?

Cancer?

Because we humans walk erect and on 2 legs, this is already really hard for the body to do and so we sweat all the time. If we had fur/hair everywhere we would've been walking wet towels.

rats are also capable of bipedal motion

We do it all the time. Much animals can stand on 2 legs, but we're the only ones who walk all our living days on them.

we also know how to sit down

It's not entirely difficult to stand on two legs. The reason that we stand on two legs is because we evolved to standing higher and stretched our legs so that we could walk upright and use our hands. the only reason that most other animals stand on fours is because their bodies are shaped to run on all fours. meerkats spend most of their time on land upright, as do most monkeys, birds, kangaroos, etc; standing by itself doesn't make you sweat unless you're fat or unhealthy and we've made up for the lack of hair by inventing clothes. your argument is invalid we have less hair because we've evolved to live without it and survive by our own means.

Just like i said.