The reason why they have dance performance / dance practices is that they perform these at live shows and concerts. Even before TIK TOK, fans used to like copying dance moves at night clubs etc. especially iconic moves (mainly the chorus section ) AND there are heaps of dance studios that allow "normal people" to learn the choreography. With the internet now, artists would also have promotions like dance cover contests all over the world, so they need to see all the choreography to learn it. Admire those people who can just copy off a screen :)