Because it's not a comedy. It's a coming if age drama and a good one at that. Movies are promoted as comedies when they not because the pr people know that it is an easier sell. So if you didn't see any comedy scenes what did you see? What did you think of TSN? Another picture in a similar feel is The Way Way Back 2015

There are humourous parts, mostly Sutter's attitude, but it's not a comedy...

Also, people tend to think of drama's as being overly ernest, sentimental and often sad... So marketers add "comedy" as a descriptor so you know it isn't an ultra-sad movie...