Why are whales not covered in seaweed?

Much like plants on land, seaweed grows along the seafloor, far out of reach from passing whales overhead, and so rarely comes into contact to latch onto their skin. However, some species of whale do find themselves ferries for small colonies of barnacles, which attach to their skin as free-swimming larvae and develop into their hard shells as adults.

A California gray whale harbouring clusters of barnacles on its head