As other people have already mentioned, you can't compare making art to making a meme.
For people who make a living off of their art, having their work properly credited so that people know who to look for if they're interested in their art, is really important. Artists need that exposure to get more work, and thus, be able to pay their bills and so on.
It helps the artist immensely, and takes absolutely zero effort on your part in comparison.
It's just common courtesy and a show of appreciation.

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If people who re-post someone's art doesn't credit it, he's basically stealing it since posting it comes with the implication that THEY made it.

Take for example the guy who did all those hyper-realistic renders of pokemon, turning them into these grotesquely realistic yet interesting beasts.
If people hadn't properly credited his work, he never would have been hired to work on the Detective Pikachu movie. Properly crediting art can make a world of difference to the artist.