As unconstructive as this comment is, the answer is simply because they are complete morons.
No other business would get away with the shabby operations of insurance companies but we have to except it because firstly we all have to have it and secondly because we get locked into companies for a year in view of not losing any no claims bonus, and because if you cancel before the end of the year you lose most of your money.
Adrian Flux handles things better than most because they have some staff that know about cars. But insurance companies and imports are bad enough but none of them like the idea of ANY modification to a car. It implies that you think your in the fast and the furious and go around racing every night.