Because I’ve seen that movie before.
Actually, I read about the movie in one of the kiddie computer magazines in like 1984–I forget which magazine, but I had a subscription to it several years before I had a computer–and I eventually watched it at some point.
I don’t have it in the library here at Nogglestead, and it’s not likely that I’ll find a videocassette of it in the wild, but one never knows.