We know that Muggle-borns get someone from Hogwarts with their acceptance letter to explain everything to their parents.
Q: How are Muggle parents convinced to let their kids go to Hogwarts, a strange place they never heard of before; and wouldn’t they think it was a practical joke?
A: In the case of Muggle parents, special messengers are sent to explain everything to them. But don’t forget that they will have noticed that there’s something strange about their child for the previous ten years, so it won’t come as a complete bolt from the blue.
J.K. Rowling (2004)
‘It’s real for us,’ said Snape. ‘Not for her. But we’ll get the letter, you and me.’
‘And will it really come by owl?’ Lily whispered.
‘Normally,’ said Snape. ‘But you’re Muggle-born, so someone from the school will have to come and explain to your parents.’
Deathly Hallows: Chapter 33 - The Prince's Tale
Since Harry was given into the care of two Muggles, how come they got pelted with letters instead of having someone from Hogwarts coming to explain it all? Had Dudley been a wizard, he would have been Muggle-born, so it seems that Harry should have gotten the same treatment since he was raised in the same household. Is there a valid in-universe reason for this? Or is it simply a plot device?