Large fleet operators tend to self insure rather than having traditional auto insurance for what it's worth.

If you have a large fleet, say getting in 5-10 accidents a year, you can't buy a policy that's going to consistently pay out more than the premium, at least not one that the insurance company will be willing to renew. So economically it makes sense to set that money aside and pay out directly, perhaps covering disastrous losses with some kind of policy.