It's a British effing trope. It was a magical kidnap-cold-poop-hungry proof basket.
When JKR was writing the first book it wasn't supposed to be a literary masterpiece, it was a book about magic and she wrote it in a coffeeshop.
No one leaves babies on doorsteps. They did it on Call the Midwife once and the people in the show were hysterical.
Seriously, the only other option was to have him grow up at Hogwarts and he would have been playing with a baby basilisk by the time he was four.