This is my guess.

A game is different from a film because you can make decisions in a video game. It is observing the consequences of those decisions which makes a video game interesting. But it can also be interesting to see what would have happened if slightly different decisions were made, or if they were made in a slightly different context. That's why people replay games. A film based on a video game is essentially just a replay of the game that you are watching, where the characters make slightly different decisions.

A film presents a single narrative. When attempting to adapt it into a game, designers must consider where choices can be introduced, and what consequences those choices are allowed to have on the narrative. They are constrained by the source material in this regard, because the player already expects that their choices must progress the plot in a manner similar to what was presented in the film. So it's much harder to design choices which are meaningful to the player. A film that presents only one narrative inherently constrains the amount of artistic freedom available.

When a game is designed based on a film (or equivalently, a book) that doesn't have to be faithful to a specific plotline, but merely uses the systems, settings, and thematics of the source material, the much more available artistic freedom allows for more interesting plots to be written and more meaningful gameplay.