That's basic physics.
The AM and FM radio is tuned to a frequency that's present and the everywhere, and can be picked up by as many devices (1000's and 1000's across the country).
The frequency of the router is everywhere (in house) also, but the contents at any moment are meant for only one device. And when that's being sent, all other devices don't receive anything (or better said: they do receive it, but they skip it because they are not the receiver that is addressed).
Signal strength is something else than the number of bits the signal contains. The radios all use the same waves at the same moment), the WiFi device has to wait till for bits. And the more devices are using the router, the smaller part of the bits it sent are for you.
An AM or FM radio signal can be picked up by hundreds of radios in a home without affecting signal strength for each radio.. But, as per my local ISP's TV commercial, if a home has a router and then multiple devices (PC, smartphones, tablets, etc. etc.) receiving signal from the router, somehow data for each device uses up the bandwith. My question... how is this different from a radio signal (unlimited) while wireless devices seem to "suck up" and slow down wireless data coming from a router? .