Because you have one of the cheapest. I have motorbikes whose policy costs more than the one for your car. Saving 25k per year on a policy doesn't sound outrageous.
But it's offtopic anyways. To be a bit more ontopic, OP: most policies in Thailand as previously mentioned are on the car no matter the driver. Insurance companies do offer discounted policies for named driver only so if you want to be sure either check the policy (needs reading and understanding of Thai) or easier: have your GF ring the company.