"Yes, I thought it was very good." Why "thought"?

Here is a sentence from "Essential Grammar in Use" book by Raymond Murphy:

Did you enjoy the film ? Yes, I thought it was very good.

The correct answer in key section is "thought", but why not to use "think" instead:

Did you enjoy the film ? Yes, I think it was very good.

It looks like you thought in the past and have another thought now.

It does indeed mean you had that thought in the past, while watching the movie. It doesn’t automatically tell us anything one way of the other as to what you think now; and I think the natural inference without further information is that probably you still think it’s good.

In some contexts, particularly, if the ‘pastness’ is unexpected or emphasised, one can indeed infer from the use of past tense that something has since changed. For example:

A: What do you think of Obama?

B: Well, when he was campaigning, I thought he was very good.

The implication here is: “…but I don’t like him so much now”. But this implication isn’t automatic from the use of past tense; it comes from the context, from the fact that if B still likes Obama then a more natural response would be just “Yes, I do.”

But in the OP’s question, the tense of the answer is simply following the tense of the question (the most natural grammatical thing to do), so there’s nothing particularly suggesting any change of opinion.

On the other hand, the present-tense “I think it was very good” would also be fine in this example, and would give the impression that you probably also thought it was good at the time you watched it. Although again, in some contexts an emphatic use of the present tense could imply that things were different in the past: