Polarization of Radio-Frequency Low-Noise Amplifier
The clue is in the title... "The design of ... integrated circuits".

You are correct that you could set the bias voltage with a resistive divider ... IF you knew what the correct voltage was.

But you don't ... 

Look at some MOSFET datasheets : note that there isn't a specific value given for Vgs(at some current) e.g. Vgs(th) often refers to a threshold current of 0.1 mA - but a range, say 1V to 3V.

All you can be reasonably sure of is that M1 and M3 require (reasonably closely) the same voltage, as they are made in the same process, and at the same temperature, by virtue of being close to each other on the same die.

If it helps to think of M3 as an analog computer calculating the correct voltage for Vbias, that's pretty much what it is.

EDIT : the point of Rbias is to isolate the RF path from all the parasitic capacitances around M3, and to attenuate any RF signals reaching M3 (which would attempt to amplify them onto Rref...) M3 is supposed to be DC only, and you can make R3 as high as you like because Ibias is practically 0. (If the book recommends kilohms, that is about keeping the physical size down)