It doesn't matter.
DIVERTIDO translates to: funny , amusing , amused , enjoyable , diverting , merry , jolly , droll , comical , comic.
It means all those things.
To understand divertido one needs to mush all those words together and work out what divertodo means as an idea which includes all of those words at once. Not seperately.
There are studends here are confused in that that they think we are learning to translate. And that idea is completely wrong.
What we are supposed to be learning is Spanish. And involves thinking in Spsnish where English is not a part of it.
A problem is that "divertido" can be translated to: funny , amusing , amused , enjoyable , diverting , merry , jolly , droll , comical , and comic.
So there could be someone here who could just as well ask: "Couldn't it say, 'A comical film?'" And another could ask, "Couldn't it say, 'A merry film?'" 'And another ask, "Couldn't it say, 'A droll film?'" And so forth, on and on.
A problem is that there can be many different possible translations for verbs and Dulolingo cannot be be expected to utlize them all.
There are likely even more verbs possible than just the ones I listed above. Perhaps a lot more.
I have seen how some students seem to like to think it is their duty to test out how far out they can reach outside what is most commonly said, while Duolingo is only concerned with what is most commonly said. So, think, MOST COMMONLY SAID for your translations as far as verb usage is concerned. Duolingo does not need to include anything else. And most commonly doesn't.
I think next time I see this sentence I'll try "A diverting film." That should work, but one never knows what's in the data-base until one tries something different. It means an entirely different experience than "a funny movie," however, which is how I describe comedies! ;-)
"Fun" is a noun not an adjective. "The movie was fun to watch" is English. "A fun movie" is American. English is a bastard language but worth trying to keep as pure as possible even though there are more English-speaking North Americans than there are English-speaking British. Unfortunately, common usage will make it become correct. I am right, aren't I? See? "Aren't I" isn't grammatical but has become acceptable. You wouldn't say, "No I aren't". It should be "Am I not?"
If it has become acceptable it is grammatical in whatever dialect accepts it. Some things will be grammatical in some regions of the US or UK which are not elsewhere. The idea of "correct grammar" is usually a fiction developed by people who are language prescriptivists. A hundred years ago, many things that are very correct today were quite incorrect then.
According to this dictionary, "divertido" means, fun, enjoyable, entertaining, amusing, funny (in context of "causing laughter"). https://www.spanishdict.com/translate/divertido.
This dictionary adds "pleasant." https://www.wordreference.com/es/en/translation.asp?spen=divertido
Thus it means more than just "amusing". EugeneTiffany makes a good point above.
This may help: https://community.dur.ac.uk/m.p.thompson/adjectives.htm or in less technical language: https://www.thoughtco.com/adjectives-in-their-place-3078145
Because in Spanish, qualifying, or descriptive adjectives usually come after the noun to which they apply. For more detals, see https://community.dur.ac.uk/m.p.thompson/adjectives.htm