Why do you think beauty exists?
Give it your best guess.
After you think of an answer I'll tell you.
Hmm, I'm gonna say that the pagan gods who designed humanity (hellenic pantheon, norse pantheon, roman pantheon, celtic pantheon etc etc) enjoyed acts of devotion/devotional offerings (music, art, dance, literature etc) and thus gave humanity their concept of beauty in order to enable us to make them, and encourage us to want to make them.
Any objections?

(also, for the record, if you want this argument to be better, a cloudy day isn't the best pick for 'proof of beauty.' Okay, going to be controversial on an FE forum, but I'd suggest nebulae or deep sea views, things that humans would never normally come in contact with and yet we have a reaction to. In all seriousness, those are the ones where the explanations become trickier. vs 'Why would humans have a positive reaction to a clear, safe open space?' and 'Why would humans feel at peace at sunset when they need to sleep?' etc are pretty easy questions)