Because they're toddlers!
That being said, by two I might be a bit concerned about compulsive *eating* of sand, although I'm not sure what you can do, other than keeping them away from it. Certainly saying, "No, sand isn't food. It's yucky" while making a face might help. I dunno. My dd2 ate sand pretty compulsively when she was a very young toddler (under about 14 months or so), but while she is still pretty oral as 2 yr olds go, she no longer does so. Maybe a taste now and then, but she is becoming aware of things that are socially acceptable to eat and things that aren't. I'd probably keep an eye on it. If it's the only weird thing they do, I'd just chalk it up to todderhood, but if they eat other non food items or such, I might mention it to the doctor.