DC Generator Starting

Why is it that if the output of a DC generator is shorted, it will not build up?

When you short a dc generator, you are locking the rotor, electrically.

All generators are technically DC. When you short its output, you are shorting out one winding across the commutator; all other windings are either open. On an alternator (single phase) a short is also expressed across one winding, but it is the only winding on the rotor and is exposed in more that one point to the magnetic field. In the generator there are as many windings as number of commutator contacts / 2. When they are open-circuit, they have no (real) influence on the stator's magnetic field. To answer your question, No.