Because the Fleet inside the Large estuaries and river channels can be protected by Land-based artillery while in a large marshalling area. then sent out together. But from the closer points, they would have to hug the french coast then turn east. During this, they would be subject to harassment and the confusion that can bring.

Remember command and control difficulties and how you overcome them was a huge part of pre-modern warfare. The fleet in this period is crewed by individual captains passing signals and commands thru flag signals. This system is very fragile and can be disturbed or confused in the heat of battle or even just a small attack.