Regular car hire firms get their money up front. The type of firm referred to in this thread knows that they won't see a penny until the end of the hire period (which could be many months or, as in this case, years).
Further, they know that if the driver (rather than an insurer) has to pay, there's a high risk that they might default, e.g. by declaring themselves bankrupt, as may well happen in this case.
So, just like lenders who offer money to 'high risk' borrowers, they're forced to charge much higher rates than are available through normal car hire arrangements.