Why is having grandchildren so rare?

I rarely have grandchildren. Which leads to me picking my youngest son and hoping he can marry someone younger and father a child at 60. Lately I’ve been waiting to have kids until my late 30s and early 40s so that when the parent dies, their kid is 40 and still has time to face a child. It’s just frustrating. I have THIRTEEN kids right now and not one has a child. What are the odds?