I am thinking of blowing a few bucks on the 200mW 2.4 setup from HobbyKing to test my theory. My phone is 2.4 and my wireless router is 2.4 and neither causes issues with my Futaba 2.4.
Thurmond
As I understand it, it's because all of those use different systems. Phone and WiFi use a fixed frequency, (2462MHz on channel 11 which is set by default) and use a collision detection system to transfer data (They listen until nothing else is transmitting, and then they send their data). Radio systems (Tx's) use scanning, which is similar (They scan for a free frequency and use that one).
However FPV systems don't listen or scan before transmitting. They just send out the data like they're shouting through a megaphone. Then the Rx does the same, it just takes everything it can hear, it doesn't check if it's the right thing.
So your FPV Tx could jam WiFi or phones, while phones or WiFi can mess up your video signal.