Not all movie adaptations are well-received. A movie adaptation that receives negative reviews could create negative associations with the book as well. Someone who sees a movie adaptation and hates it is less likely to read the book than someone who does not already have that negative association with the book.
Another counter argument is that many people who see a movie adaptation are not interested in reading the book because the plot has already been "spoiled" by the movie. Generally, movie adaptations are faithful to the plots of the books they are based on. Even if there are some differences, major plot points and twists are often kept the same. Someone who watched the movie adaptation and already knows the ending of the book may not be interested in reading the book, since they already know what will happen. Thus, movie adaptations do not necessarily make the book more popular.