That's because most languages allow arbitrary IO in all functions. You can achieve this in Haskell too if you want, by just using IO as your monad everywhere.

That's because most languages allow arbitrary IO in all functions. You can achieve this in Haskell too if you want, by just using IO as your monad everywhere. But most people don't do that because then it becomes really hard to reason about your code, so spending the extra time propagating a MonadLog constraint up your stack is actually worth it.