Why aren't starter circuits protected?
There's only any point in trying to protect a circuit with a fuse if there is enough headroom between the operating current, and the fault current, to ensure that the fuse won't blow in normal operation, and will blow under fault conditions. 

Unfortunately, once you have included all the tolerances, there is no current level you can choose that is guaranteed to avoid nuisance tripping and still have a reasonable chance of working, should the starter motor be locked up for instance. A conventional starter motor is series wound, in order to pull a very high current at start-up.