Its about reliability of the car. The teeth that are on the bevels need to be of a certain pitch to be reliable in plastic (or most material) The sizes of the bevels have to be a certain size due to them fitting on shafts that have a certain diameter, so that also forces some sort of compromise. Also a lack of teeth can make the car not be that efficient and quite notchy.
I remember back in the early days of TC we used to sometimes file teeth of gears in the TA01/02 gearbox to reduce the IR to help gearing.. it kinda worked but the gears didnt last for long and the car was not as smooth to drive.
Also the way that the motor is mounted in the car ensures larger gears are needed. For example the A700 has an IR of 2.55 in the same configuration as the TB04 and an IR of 1.9 (ish) for the classic layout.