Who says "fun" can't be an adjective?

‘Fun’ can be used as a noun, an adjective, or verb. But I could not find the case as a verb. And people usually use ‘fun’ as a noun or an adjective. For example, 'It's a fun story.' This 'fun' is an adjective here. If you use it before a noun, usually it is an adjective. And 'The party is a lot of fun.' This 'fun' is a noun here. So the meaning is enjoyable or entertaining.

At the comparative form, ‘fun’ changes to ‘more fun’. Someone would use ‘funner’, but it is wrong. Likewise, ‘fun’ changes to ‘the most fun’ at the superlative form.