The wavelength of the signal the cellphone uses ranges from 16.8" to 4.5", so the antenna can be quite small. The antenna for 700MHz is really about the smallest resonant antenna they can put into most cellphones (it would have to be 4.5" long) and since there's no ground counterpoise under it (like sitting it on a car roof vertically), it's pretty inefficient. The antenna for FM radio stations has to be much longer - at the bottom of the band, 88MHz, a tuned antenna (as opposed to a random length of wire, which is terribly inefficient) has to be 33.5" long (again, it loses efficiency because there's no ground under it - and the ground has to be as large as the antenna, so a 3 foot diameter circle or larger - a car roof will do, a fender is much less efficient). But the best you can do with a cellphone for FM radio is use the earphone cord as an antenna. Making an antenna that's 33" long electrically and 4" or 5" long physically would make it so inefficient that you'd have to be within a couple of miles of the transmitter to receive anything.
For AM radio, the only saving grace is that the wavelength is so long (0.11 miles to 0.34 miles) that almost any piece of wire will pick up some signal from a decent distance from the transmitter, and at night, signals can "skip" a few thousand miles even if the receiving antenna is only a few inches long, due to the way low frequency signals reflect off and refract through the atmosphere.
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