Choosing where to put validations?

Consider this snippet from the "Getting Started" guide:

module Web::Controllers::Books
  class Create
    include Web::Action

    expose :book

    params do
      param :book do
        param :title,  presence: true
        param :author, presence: true
      end
    end

    def call(params)
      if params.valid?
        @book = BookRepository.create(Book.new(params[:book]))

        redirect_to '/books'
      end
    end
  end
end

Notice the validations on title and author, which live here in the controller action. My question is: Why are these validations on the action params rather than on the Book entity? That is, assuming the validations were on Book, you could write something like:

def call(params)
  book = Book.new(params)
  if book.valid?
    @book = BookRepository.create(Book.new(params[:book]))

    redirect_to '/books'
  end
end

and just get rid of the params block altogether. This seems more natural to me, and would promote easier re-use of the validations across different actions.

Are there advantages to the params method that I'm not seeing? Are there drawbacks to placing the validations on the Book entity?