Because perfect information makes insurances useless. Insurances work because we know that most people have roughly the same chance, plus or minus a bit, to get royally fucked in life by disease, acts of God, human maliciousness and all that. That means that when someone gets the rough end of the stick, the entire group helps to pay to keep them on their feet.

If we have perfect (or near-perfect) knowledge, what would insurers do? Well, they'd charge you for exactly how much you're going to cost, plus some administrative costs, plus some extra. At that point, everyone's better off not having insurance.