That's because iMessage is not SMS, it's something Apple invented that doesn't work well with what the rst of the industry uses - SMS.
It's a problem known to everyone except the people at Apple who refuse to acknowledge that the rest of the world not changing to fit its idea of "right" is a problem. One of these days, Apple will modify iMessage to work with everything else (And we'll have world peace, all your children will be geniuses, illness will be a thing of the past ... Don't hold your breath.)