Why is the sunset beautiful then?

Yes, you mean why beauty has to be utilitarian :)

A better question would be "why beauty exists at all?". We could theoretically survive without it, we have other systems driving us towards something (dopamine, hunger, etc.)

We tend to assume our brains as perfectly laid machines, but we know our meat computer is anything but perfect. What if it is "interference" from nearby systems? Maybe our sexual attraction subsystem shares some neurons with recognising striking sky colours and we find them beautiful, with no real evolutionary reason.

That would explain why we do or like many things that are not necessarily useful to survive. And this is also my pet theory of consciousness: it is just a side effect caused by our pre-frontal cortex and ability to simulate future events. Those mechanism resonate together to create what we call "awareness of the self"

I sometimes have abstract dreams with an irregular surface that slowly morphs into different shapes. It goes from being smooth and flat, to irregular shapes (that I find disgusting) and back into fractals or regular geometric shapes (that I find pleasant). The change from nice to ugly shape is evoking very strong disgust.

I think there is something to the idea that low Kolmogorov complexity is pretty.

I've also had a weird esthetic experience commuting by a bus once - it was a sunny winter day and along the road for dozens of kilometers there were power lines with unusually thick wires. There was a lot of contrast between white snow and black wires and they went up and down in a regular but slowly changing pattern that was very beautiful in the moment, but it's very hard to explain after the fact :) I was amazed by it despite commuting that same route 100s of times before and after that and never noticing it or caring about it.

Yes! Synesthesia [1] is one argument for the "interference" theory I mentioned in my comment. Sometimes the wires get tangled and a sense goes to affect a different part of the brain. A strong dose of hallucinogen will surface this behaviour even in people that are usually unaffected.

I have a form of light synesthesia [2], where I intuitively see the concept of time and age as a three-dimensional line, that slowly morphs as I grow older. Whenever someone says "I'm x years old", I subconsciously place them somewhere on that line. (Now that I think of it, years too have a particular three dimensional structure in my mind)

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1: https://en.wikipedia.org/wiki/Synesthesia

2: https://en.wikipedia.org/wiki/Number_form

If it wasn't then we wouldn't get little baby sunsets, now would we? (less facetiously, the sunset itself isn't beautiful, it's the act of sitting and relaxing with enough of your core needs met while the sun goes down for another day that's beautiful.)