When with State Farm, I had my old car covered by them, the reason being I had an umbrella policy. The story I got from my agent was that to be qualified for the umbrella policy, all car policies and the house policy had to be through State Farm. Not sure if that was true or not, but the agent was pretty insistent. Maybe that is the case with the OP.
Anyway, State Farm decided to not renew our homeowners policy. The reason given made no sense, but rather than fight I moved on and am now with an insurance carrier that specifically told me to get a policy for my Plymouth through one of the usual collector car specialists and simply give them a copy of the sheet showing coverage (which had to have certain minimums for the new umbrella to be valid).