There are two reasons

1. The market is saturated. Everybody pretty much already has insurance and they only shop for it when they have a reason to. So you want them to have your companies name on their mind when the time comes. The biggest source of new customers are people who switch from someone else.

2. GEICO was having name recognition problems when it transitioned from covering government employees exclusively.(Government Employees Insurance Company) This is where the lizard mascot came from. It was a huge success and other insurance companies followed suit. What we have now is a sort of arms race where all the major companies spend ridiculous amounts on advertising, but nobody wants to scale back for fear of being buried by their competitors ads.