"Errors & Omissions" liability insurance policies generally have a dollar limit on liability covered, so a company offering an unlimited guarantee would basically be covering the difference between that limit and actual damages to a customer out of pocket.
I have no idea how to change this reality, though... If you're an insurance provider, it would be extremely complex (more so than normal insurance calculations) to figure out what that liability really costs the insurer to cover in order to properly price a policy. Because of that, there is a lot of risk that even if a policy is written the insurer could have significant un-hedged risk if they calculate incorrectly, of if they're not conducting regular (expensive) audits of the insured company to make sure the risk of liability hasn't increased since the policy was written.
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