There are a number of reasons, one of the biggest being that your jungler needs the support and security. Generally speaking, the team that brings more people to a fight is going to win. 5 people catching 1 or 2 alone is bad news at any point in the game, even lvl 1. So even if your team isnt looking for a first blood, it makes sense to stick together until creeps hit the lanes. We have a huge advantage if our 5 man squad DOES catch someone, and were at a huge disadvantage if we aren't together and one of us gets caught.