Asking “why” can still be a legitimate question, and “for fun” can also be a legitimate answer.
I treat projects differently if they want to launch a product, they want to replace an established open source tool, done for fun for themselves, or if it’s a hobby project.
The complaint isn't about being asked why, it is about "for fun" not being acceptable.
Follow up questions are totally cool but the context is different, right?
If it isn't acceptable then there's a negative tone and questions are focused on utility and usually them "trying to help you" find utility.
If it is acceptable they ask you about your interests and what you're learning. Sometimes that can turn into utility but that's more natural.
It's a lot about culture to be honest. Some people are just toxic and if things don't make sense in their heads then it doesn't make sense in any head.
'For fun' is a perfectly reasonable answer. But sometimes the answer to 'why' comes after you do it. I'm sure that everyone has learned something without realizing its utility until the end. Nobody is obligated to answer that question, unless you're pitching something.
I treat projects differently if they want to launch a product, they want to replace an established open source tool, done for fun for themselves, or if it’s a hobby project.