Any insurance underwriters out there?

I run two cars.  I am the only insured driver for both of them meaning (unless I am missing something) that only one of them can ever be being driven at a given time.  My question is, if insurance premiums are all about the risk of making a claim, why do I have to pay the 'full' amount for both cars.  Apart from fire and theft, how is the risk of me making a claim increased by me having two cars?  Or three?  Or even four?