Have you ever placed your mobile phone next to, say, the speakers on your PC, or your bedside clock-radio? Ever heard the "yada-yada-yada" noise just before the phone rings? Well, I guarantee you that your computers speakers don't even have a specific "frequency", yet somehow the mobile phone interacts with them, doesn't it?
Now imagine the emergency exit slides on the aircraft, or the fire-detection system, or the generator control unit, or the fuel control unit, or the... Do you really want to be turfed out of your nice warm aeroplane onto the cold tarmac of the apron just because that phonecall was so very important that it caused an evacuation? I won't even consider the effect the phone might have in flight on the autopilot, the navigation system, the pressurisation system, ...
As to why the mobile is the guilty party not the ground station, well, it's all a matter of distance. Double the distance from the source of the interference and you've quartered (inverse square law) the effect. The mobile phone mast is at least a kilometre away. The mobile phone itself might be fifteen centimetres from the cable buried in the sidewall of the aircraft. Need I say more?
I only fly light aircraft, but I have forgotten to switch off my mobile once. It rang in the baggage compartment of the plane, and the GPS wandered off by about 15 miles. Fortunately I'm only a private pilot so I fly by looking out of the windows - but what if I'd been on an airliner relying on my navigation equipment while flying in cloud?
Fly safe - switch the mobile off, and keep it off until you're in the terminal building.
MD.