The computer and the internet have opened an entirely new spectrum of coloring and drawing. Adults now make very sizable livings creating, drawing and coloring pictures for computer animation including video games, movies, commercials and items most adults would never consider, such as some high-quality information websites. With this option available as a full-fledged career for adults, it is no wonder that the tools and formats used in this style of artwork have carried down to children, albeit on a smaller scale.
If you really want to get right down to it, adults love to color as much are their kids do. If you dont believe us, think about all of the artists that do oil and watercolor paintings, those that do pen and ink drawings and watercolor pencil drawings and artwork. This is simply the more appropriate and acceptable way for adults to continue coloring as we all did as kids. This is partially why both adults and children alike really enjoy having SpongeBob printable coloring pages available to them to color on.