Selling artwork with copyrighted characters?

I’ve noticed that many artists sell originals, prints, stickers, patches, etc of copyrighted characters (eg, Batman and Robin, Bart Simpson).

Legally speaking, how is this allowed? In the case of Batman and Robin, they don’t enter the public domain until 2035. Bart Simpson enters the public domain in 2083. So I’m just confused as to how artists are able to sell art with these characters. Anybody know how these artists are able to sell such artwork?

I have searched this topic on the sub and haven’t found anything relevant. So was just curious to hear others’ thoughts.