This was nothing like your interaction though. Asking why doesn’t mean they don’t acknowledge the other person doing it for fun, it’s just that they are curious if there’s a reason.
-Haha, why?
+Found it cool and felt like it.
-That’s cool, I used to play X when I was younger but…
It can also just be a conversation starter, nothing wrong with asking why.
You don’t think there’s a difference between “Why the fuck would you do that, you nincompoop?” and “Haha, why”?
I do, which is why I said that your example is not like the one of the comic.
I think the face in the comic convey quite well that “haha why” was not a “wow, I’m interested” but “the fuck you doing, bro?”. Especially since it’s literally prefixed with “what the hell are you doing?”.