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They could. It's very hard to enforce a cartel like that when there are a large number of competitors. It's a prisoners' dilemma with dozens or hundreds of participants. It only takes one defector to break it.

If you've ever shopped for car insurance, it should be pretty clear that there isn't a cartel holding prices high. Prices differ substantially across insurers, and are influenced by many other factors as well. Premiums are much lower if you have a clean driving record and no claims, or if you drive a car that's cheaper to repair, or less likely to cause injury, or you're of an age/gender with less propensity to crash, or live in an area with less automobile-related crime. Why would they give you lower rates for these things when they could just keep the premiums high and collect more profits?