I've asked for higher coverage, but it was denied.
Just Guessing here, but I suspect the reason you were denied is you don't have enough vulnerable assets to justify you having higher coverage. Often times a primary residence and 401K are not considered vulnerable assets. I suspect Future assets most attorneys are reluctant to go after as it is difficult to collect, but should be considered in the Asset calculation.
Insurance companies don't want to pay out anything more they have to. If you only have $100,000 in assets they can sue for then they don't want to insure for $1,000,000. Doing so just insures that they will be aggressively sued for the maximum amount you are covered for, with every attorney in town fighting over who get to go after it.
Now if you have $1,000,000 in assets and only $100,000 policy the Attorneys are going to say screw taking the $33,000 he and the injured will get from the insurance policy and let's go after the $1,000,000 in assets so he will get $333,000 or more if he wins.
BTW the majority way damage payouts go is the injured gets 1/3, the Attorney gets 1/3, and the injured insurance company takes 1/3 to cover their expenses, Assuming the medical expenses were at least 1/3 of the available payout.
For this reason the best thing you can do for your passenger is make sure they have medical insurance, as it will take the hit if they get injured, even if you had no insurance.
For your protection, you just want to make sure you have enough insurance to make the math simple in the event of an accident. Enough Insurance to take the insurance money and run, instead of them or their insurance company suing for your assets. When they take the insurance money it is on the condition that they won't sue the insured, if they decide to sue then they forfeit the insurance money and that money will likely be used for your defense, but it is not an asset they can go after.
I also agree with previous posters.
If you fly in good VFR days, just do a basic ride flight above 1000AGL then your passenger is about as likely to be injured driving to the airport as they are flying with you.
Of course all of the above should be checked with an Attorney, Financial Advisor, and Insurance agent ( I am non of those) as the above may not apply to your state, financial situation, or insurance and might just be pure hog wash.
Brian