Before the meeting with Zeff, we only knew that he was a ship's hub and did not know the importance of food the more he threw the leftovers .....
It was not important to show where his ability to cook was born, because the character owes everything (his philosophy, his way of fighting) to Zeff, not to the Vinsmoke, not to the mother, but only to him ....
The Vinsmoke and the story that Oda has sent Sanji to WCI do not work because they simply do not get fired at what Sanji showed up to then.
Otherwise it would be like asking where Brook took the passion for music. But is it important? No because Brook owes everything to his experience with the Rumbar, or wonder why Franky even before Tom was so skilled at building objects.