I’m not sure what your confusion is. Kids love watching their favorite movies over and over again. Plus this was 1977 when there was far less media to consume, probably 5 or 6 channels on TV, and with no home video who knows when you would be able to see Star Wars again after it left theaters.

Plus going to the movies in the summer was one of the things to do when I was a kid in the 90s, I’m sure the 70s were similar in that regard.