A couple of reasons are cost and being distracted. If you give kids a computer, they'll definitely prefer to dig around in it doing absolutely anything besides

A couple of reasons are cost and being distracted. If you give kids a computer, they'll definitely prefer to dig around in it doing absolutely anything besides school work, no matter how boring the other things on the computer are. And plenty of poor kids can't afford to just carry a laptop around every day, which means risking losing or smashing it. Schools can have them but they won't be instantly available - there'll be 10 minutes of class wasted getting out the laptop cart and signing them out and collecting them back in, and dealing with kids fighting each other over them. It's a massive pain. I've done that and it's not at all practical for daily use. Then how do they do their homework if the CAS computer's at school? I've heard of schools using Chromebooks and Ipads, which might address some of those problems, but not as well as a basic scientific calculator.