Starter motors are only designed to run for a few seconds at a time. If you run one at significant power levels for more than that, it may quickly burn up. (people kill starters in their cars by cranking them over and over for too long). You can add cooling systems but that's more work, money, and materials (and weight and volume added to the whole bike).
They also don't usually have bearings, but only bushings, so they also don't have a very long lifetime. You may be able to replace the bushings with bearings, but that's more work and money.
Most of the motors on a car are either intended for low power or low duty cycle, or both. Some things like alternators can be converted to motors, and has been done successfully and documented around the web many times, but most things can't be used straightforwardly.
A powerchair motor with gearbox would be a better option, as far as brushed motors go; they can be had in pairs for free or cheap if you find someone with a dead-battery chair they don't use anymore. Sometimes they're cheap at thrift stores. You can look at my CrazyBike2 thread's early posts for how one can use one to run a bike. Their disadvantage is weight (much of it from the gearbox), but if you want torque...I stopped using them because I kept ripping stuff apart and bending things on the bike. :lol: Could've been fixed by redesigning the frame to deal with it, but at the time I didn't want to deal with it anymore.
There are brushless powerchair motors too, but they're harder to find (I'm still looking for a second one).
It's usually cheaper and easier to find old ebike parts these days, than to adapt other motors for use on an ebike. (coming from me, that's probably quite a statement :lol: )