The main reason is that most babies have tiny tummies that don't hold a lot of food. Spacing out the feedings ensures that a baby fills up on the good stuff (formula or breastmilk) instead of solids, but then has a small appetite for solids a bit later. It can also help a baby eat more daytime calories. Even most toddlers eat every couple of hours because their little tummies don't hold much food...breakfast, snack, lunch, snack, and dinner. When Tyler was a baby and I tried to do breast/bottle and solids all at once, he either wanted nothing to do with solids or spit up a ton. So it didn't work for us.
Of course, every baby is different. Some babies are totally happy having both bottle and solids at the same time, so it comes down to whatever works best for you!