All the comments about technical details and practical details...
Are we forgetting that when dealing with millions of people, plus bureaucracy, in a potential death risk, it's worth being slow and cautious about relaxing former safety rules.
That was my understanding, that once upon a time it was legitimately feared that mobile phones could caused accidents (and thought they had indeed caused one). So, besides the other issues people have highlighted in the comments, to walk back from the safety rule of turn-them-off is a slow process.
Commercial air travel is not known for going, "ah, it's probably fine, don't worry;" except in the case of emergency exit door bolts.
... Please do follow the rules, and do what the cabin crew ask you to do. Otherwise, even if you don't directly endanger the plane, you make it harder for them to do their jobs and keep everyone safe.