after all a credit card limit is in place to cover what the card holder can afford to pay back. so it doesnt matter if its a car, or a bitcoin. the credit limit secures the debt risk limit.
the real reason is to avoid costing the banks lots of money because of chargeback scammers.
they wont say that though because it would be just advertising to scammers that performing a chargeback is an option to get bitcoin and money back