It's actually about the logistics of bacon. In-N-Out puts quality before all else. Most fast food places use precooked bacon which is terrible bacon and anti their culture of quality. So that leaves cooking the bacon themselves. Now normally, a restuarant will parcook bacon fresh at the start of the day then finish it to complete an order. While this works for many restuarants, the volume of burgers each In-N-Out sells daily is astonishing and the amount of bacon required is just too much to warrant adding it to their menus.