Because unless a meme gets some upvotes, very few other users will ever see it. Getting upvotes is the only way to assure a meme doesn't go straight into the abyss of page 199. There's not a lot of point in making memes unless some people see them
The internet is an abyss that's true even for the most popular. They are only preaching to their own choir. It's public speech yes, with no real audience.