That is true, but nobody knows for sure how much a movie will make. Generally you know the more you invest into advertising the more people will want to see it, but how much return exactly every dollar spent on advertising will generate won’t be known beforehand.
The job of studio executives is to estimate this, and based on that make choices. This is based on some amount of objectiveish data (such as market research and comparative trends) but also has a large component of guesswork. And this guesswork is where his personal preferences can colour his decision making.
In other words if he would know it will make a lot of money he would release it for sure. You are right on that. But it is not known, and perhaps the objective indicators are ambigous or borderline, that is where his personal preferences might make a difference.