Because there have never been and never will be real studies to determine whether this is actually a dangerous thing or not. The fear is that your device emits a signal on a frequency that will interfere with flight systems and cause problems or a crash.

Starting in the early 2000s, they would make you turn off ALL electronic devices just in case and you weren't even allowed to read a personal organizer or perhaps an ebook reader. It took them 10 years or so until it was finally decided that your Bluetooth headphones weren't going to crash a plane, and now they all have WiFi on board anyway.

There may come a day when they stop caring, but for now, it's all restricted out of fear/paranoia rather than being known for sure to be a problem. As someone who has flown for decades, I have observed that many (MANY!) people have really never cared to follow these instructions and we have yet to hear of a crash being caused by electronic interference.