Serious question...why would anyone not do agreed upon value for a 350-400k car? Is there something I'm missing? if you get rekt, and their insurance only covers xyz, or you paid 400k cash now the resale value is 250k etc.
Many reasons.
First, the agreed value only comes into play if the car is stolen or totalled. The chances of that are extremely slim and more than likely never going to occur.
Second, Let's assume you bought the car used and paid 320k for the car. If the car is stolen or totalled the ACV may be 275-300k (just giving an approximate as an example). Is the difference between the increase in premium for agreed value worth the difference in ACV value. If you have a loan or lease, than it may be worth it or necessary but if you paid cash than it may not be.
Third, triggering the total loss on an agreed value car is different than triggering the total loss on an ACV car.
Fourth, you need to pay attention to what is covered, not covered, specifically excluded and deductible. on an agreed value policy. Most of these policies are not standard policies that would be regulated by the state and as such may have odd exclusions. A lot of agreed value polices have a 2% deductible or some other amount (different than your standard $500 deductible). Using your example at 400k that is a 8k deductible. Add that to the increase in premium you pay and is it worth it? You could go 10 years without your car being stolen or totalled and if you add up the increase in premium you paid and the deductible, you may have been better off with a ACV policy.
Insurance is all about risk tolerance. Some people self insure, Some people are fine with ACV and others want the safety net of agreed value. It is really dependant on your personal tolerance level.
Believe it or not, a large percentage of homes over $10 million have no homeowners insurance. Those people would rather not pay huge premiums and can afford to absorb any losses. Conversely a homeowner with a 500k home may over insure as they would rather have the protection and they couldn't afford to absorb any loss, should something occur.
Hope that helps some.