You could equally say "Just because math is involved doesn't make it engineering".
But I suppose it's something in between art and engineering.
With regard to "sell weird color splotches for millions" - Isn't that the same with code? You can sell a simple game or app to millions of people and become rich. The amount of money you earn doesn't have any connection to complexity.
You are right, there is a sub-gendre of entertainment software that does have similarity to art. But even there you'll never buy a product because it crashes, while a work of Art can be valuable because it is ugly
On the other hand, in code styles, languages, idioms, a lot depends on taste. One person might find Lisp code ugly, another one might love it. Some person might find some library useful as a modular object oriented system, while someone else might find it a hellish heap of macaroni code.
Artists might like to experiment with different styles, so might coders.
Also, it's not like most artists try to sell people ugly things. That's also a very small niche. Most of them simply work for companies and design the things that they are told you. Just like coders.
We could debate about this for ages, but it seems very clear that there is both art and engineering involved.
But I suppose it's something in between art and engineering.
With regard to "sell weird color splotches for millions" - Isn't that the same with code? You can sell a simple game or app to millions of people and become rich. The amount of money you earn doesn't have any connection to complexity.