The thing is, there is no reason for the PCs to know each other, or even be in the same town. And none of the starting quests involve recruiting a group of disparate individuals to do X. They all start off by assuming the PCs are already a group, but the quests are too low level for them to already be a group, unless everyone in the party already shares a backstory.Phandelver is explicitly a "first time playing" adventure. Anything after that first time requires at least a little more consideration by the GM for how the party comes to be a party. "Hired to guard the caravan" is the thinnest, weakest "background" this side of "pressed into service by the Flaming Fist."
For my part, I am going to have the PCs all connected to different towns and know people in each of them so that when they go there, there's a real reason for them to care about things happening. I am also starting the PCs off at 3rd level so they will have a bit of "heroic mystique" about them, so there's a reason for locals to ask for help.