Why do we shoot low-speed film for long exposure?

Wouldn't it make more sense to use faster film, so that you can get the exposure done with faster?

The answer? Reciprocity. See, normally film obeys the reciprocity rule. Go from f/2.8 to f/2, you can take the shutter speed from 1/30 to 1/60.

However, all films only obey this within a range of shutter speeds.

So, take for example Kodak's T-Max films. At 100 seconds, you T-Max 100 calls for a 200 second exposure. At 100 seconds, T-Max 400 calls for a 300 second exposure. T-Max P3200 calls for a 400 second exposure at 100 seconds. If you continue the curve onwards, you'll quickly discover that the 100 speed film will end up calling for a shorter exposure than the 400 speed film.

Digital cameras are different. Digital sensors don't have reciprocity, but they do accumulate noise in a variety of ways. There is often a sweet-spot that balances ISO-noise and dark-noise for a given sensor.