Is there some reason why the crank on a bike is kept narrow? I am planning on installing a motor on a bike and it would be helpful if I could make the crank (or the space between the pedals) about 5 cm (2") wider than it is now. I am not very concerned about aerodynamics.

Chainline.

I understand that the chainline must be kept reasonably straight...but what if you can widen the pedal arms so that the pedals are farther apart from each other?

Aerodynamics, a wide wider catches a lot more air and its is easier to go narrow than to go low.

Biomechanics: there is some optimum width for your foot separation, if you ride with your feet wider you may be less efficient and with heavy use, may cause some twinge in your knee or hips.

There are lots of motor configurations, some driving the BB spindle, the main chainring, a secondary chainring on the non-drive side using a 2nd chain.