Most Grandmas do have curly hair, I admit, although you'll see a few with shorter, straighter styles these days. I think it's because curly permed hair was very fashionable when they were younger. And, when adults find a hairstyle they feel happy with, they tend to hang on to it for life. So most Grandmas liked to have curly hair when they were younger, and they want to keep it that way now.
probably because as you get older, you cant be bothered straightening it so it goes curly. Or prehaps they get it cut short and permed so they dont have to bother styling it