Ron went an entire school year with a broken wand while his best friend had a mountain of gold sitting in his bank vault.
They both crashed the family car and Harry offered nothing to fix it.
Give Harry a break, he was like 13 and raised by the Dursley’s. I don’t think they taught him “if you have a giant pile of money, be sure to help out your friends when they need it”.
And I dont think Molly would have let him, the Wesley’s always made do on their own. Harry (a kid) helping out would put quite a stain on their pride.
Also, not sure they knew how rich he was. Sure he bought all the stuff on the trolley, but that’s like giving a kid $50 for their birthday, not a bank full of your parents’ life savings.
And they were in the car in the first place because Ron was helping Harry get to school.