For several reasons:

1. The effect can be cumulative. So one person by themselves leaving a device on that should be off may not matter, but many people on the plane may matter.

2. Taking off and safely landing doesn't mean there werent' issues (that good pilots managed to work around). You may be causing issues that are unreported and cause some problems, even if they don't escalate to an obvious-to-passengers problem with take-off or landing.

3. A properly functioning device may not be an issue. But a malfunctioning device can put out a lot of RF interfeernece that a properly functioning device wouldn't, but the only sympton may be the greater RF intereference, and I bet you aren't checking for that symptom on your devices regularly, are you???

(I havne't done checking for airplane-specific RF frequencies, but I know some devices are way worse at interfering with FM and AM radio and even cellphone transimissions than other devices, and sometimes it's due to design but other times it's due a defect or malfunction.)


So just because most of the time it doesn't cause a problem big enough for you to be aware, doesn't mean that it never ever ever could cause a problem that's not trivial for the pilots.

And how do you know it didn't even cause problems for you? Interference to airplane equipment during takeoff or landing might manifest (as far as you are told) only in a delay in one or the other!!! (Because it takes time for the pilots to use an alternate procedure to work around the malfunction.)