Fair has nothing to do with it.
The other driver's insurance company is not your insurance company and owes you nothing until and unless a court of law says so. Until then it's all a matter of negotiation.
Frankly, I'm also surprised that the other driver didn't get a ticket.
Here in AZ making a left turn in front of somebody and getting hit is an automatic ticket and an automatic 100% at fault.
Personally, I think the other driver's insurance company is full of baloney, but if you accept their offer, they'll pay you 49% of your damage and then go after your insurance company for 51% of the other driver's damage and you would have an at fault accident on your insurance company record.
I think you would do a lot better in small claims court. IL small claims limit is 10,000, it's cheap and informal, and you don't need a lawyer.
It's even possible that the other driver's insurance company will change it's tune once the driver is served with the lawsuit because then they have to add the cost of defending to the cost of the claim.
No guarantees, of course, but I'd go for it if I were you. You can't just roll over because some insurance company says so.