Low voltage (low enough to slowly turn or stall starter) kills starter relay contacts by having them chatter with peak or near peak current. Most 12v vehicle starter system are in no way even rated for near peak or locked rotor current service. A few contact hits at those high currents produces enough arc to pit contacts. Pits increase resistance across contacts.
It’s one thing to close contact, heavy load, light load and release vs. close then release under heavy load