Frustrated at claim stage

After paying premiums for so many months/years, I find myself frustrated at claims stage. This morning at 06h30 on N2 a stone was thrown onto my windscreen which was a very traumatic experience. I know on glass I have an excess of R1600, but just for interest sake I call an auto glass company to see what the total cost would be. I write this review in shock as the total cost of replacing the windscreen is only R2000 and I am expect to cover 80% of the repair cost, I called in asked how is it possible to be paying insurance for so long to only have 20% cover which in this case is less than 1 months premium. I am told the excess is stated on the policy I onus is the client to know, which frustrates me even moee because the onus was never on the Client to know the value of the vehicle which also carries a separate excess. My is question shouldn't an industry giant such as outsurance know that they are writing a policy which only covers 20% of glass repairs, or is intentional?!