They have to say it because otherwise people don't subscribe. I know some youtubers who never did it hut had to resort to saying it because their subscriber numbers where stagnating otherwise.
Whoring for likes/subscribes is desperate and it cheapens the hell out of your art. It may give you more likes/subscribes but there are other great things that come from artistic creation than the pitifully minimal effort required by the clicking of a button.
I can phrase it as 'like OR DISLIKE and subscribe and click the bell icon', and explain how the dislike button is the same thing (both go to 'engagement') plus a little feedback for me the creator. So you can help the channel AND try steering me away from more channel-whoring by disliking and subscribing. Hell, maybe it helps you more. Please dislike and subscribe :D