Why doesn't your home insurance cover damage from a vehicle which is not insured under auto insurance?

That is not necessarily true. That is, if an uninsured vehicle runs into your house and causes damage to the house, your own homeowner's policy will generally pay for the repairs to the house. Naturally, payment will be subject to the terms, conditions and limitations of the policy. Further, the amount recoverable will be limited to the total amount payable under the homeowner's policy.

If, instead, what you are asking is whether homeowner's insurance will cover damage to a vehicle, the answer is no. They are 2 different types of insurance covering 2 different kinds of risks. Premiums (the amount paid by the person insured) are determined by the nature of the risk(s) insured, the persons or entities insured, and the activities insured. Those factors differ between auto insurance and homeowner's insurance, so a total premium cannot be melded.