Because you generate a charge of static electricity as a result of your clothing rubbing on the material of the car seats.
When you fill up with petrol, do not get back in the car with the nozzle still in the tank. When you get back out again and grasp the nozzle, an electric spark could ignite the petrol vapour. Somewhere on the Internet there is a video clip from the CCTV camera of a petrol station showing this happening to a driver. (Maybe someone will be able to post a link to this).
It is for this reason (danger of creating a spark) that you should not use a mobile phone on a petrol station forecourt.



