Without denying the premise, I'd like to question it.

You're saying that very entertaining films are generally not considered great, and that seems like an unbalanced comparison, in that one side of the dichotomy is defined, and the other side isn't.

When I say that a movie is very entertaining, you have a fairly good idea what I mean: it's fun, it holds your interest, it doesn't alienate you, it can be watched a hundred times, and so on.

True, different people will have different definitions of entertaining, but I think they are generally much closer to each other than the definition of 'great'. What does great even mean?

To some people, a great film is one that shows artistic merit, or technical craftsmanship, or raises difficult questions, or presents new perspectives, or social commentaries, or abstract concepts. To other people, a great film is one which is very entertaining.

To some, a great film is one which makes a lot of money, right?

The word teleology fits in here somewhere. We have to ask what films are for in order to ask what makes a film great, and I don't think there's one single answer to that. Whether a film is great or not comes down to what you, the individual viewer, is looking for, or (at most) what we, the culture in abstract during a particular time interval, are looking for.

So, I think your question sort of comes down to an observation that there is a category of people who are interested in things like abstract concepts and clever techniques, and use the word great to describe that type of film.

On the other hand, there are people who like films that are fun, spectacular, familiar, enjoyable, and so on, and they also use the word great to describe that type of film. Either they're both right, or nobody should use that word at all.