Our home had lots of books, lots of discussions about books. Two teachers in the house. We were encouraged to get out the relevant volume of the Encyclopedia Americana if we had a question about something (even in the middle of dinner).
I am and have always been a voracious reader of books (all kinds). I have had since childhood an almost perverse love of books (I went into publishing), but both of my younger brothers were not big readers when they were young. They obviously did and do read (one has a master's degree in engineering). They are very intelligent and keep up on the news and science and such. They just didn't curl up with a book for 6 hours like I did. I have no idea why.
My brothers were, however, interested in learning. The issue these days, I think, is learning (and a lot of reading, too) seems to be all in the computer, rather than out in the world. I think what you want is to get them off their computers and find "real" things to do. What those real things should be, I don't know. Kids aren't allowed to do much on their own now. Do they like making things, fixing things, taking apart old toasters and radios, music, creating art, cooking experiments, learning how to grow things? They should pick a project for the summer. And yeah, one of those projects could be a part-time job or helping others or volunteering.
My mother, an art teacher, would say reading isn't the only way to learn and be curious. Also, check out Gardner's "Multiple Intelligences" theory.
ETA: Sometimes now I wish I had been pushed to not read so much and instead learn things like how fix a toilet, grout some tile, maintain the car, replace a light fixture, etc. You know, DIY.