Certainly not a requirement
There are dealers that for some reason want to pull your credit even if you bring your own financing.
In short – it’s completely unnecessary and certainly not a requirement. Usually it’s just a part of their effort to win your current financing.
If your ONLY concern is the hard inquiry, don’t sweat it. If you had all the pulls in a period of 45 days, FICO counts all as just 1 pull (see Rate Shopping).
How are your bargaining skills? If you’re not interested in the dealer’s offer, try walking away. He may back off if he’s not interested in loosing you.
Good luck