Because they use their navy well, and have access to all med provinces because it's so strong, that they can easily send an surprise expidition to take over any weak province in countries that have no navy (or a small navy that they can quickly destroy without taking enough casualties to break their chain).

Because their empire is usually widespread though, they are prone to civil wars which usually breaks them up. If their naval chain is broken for any length of time, internal trouble follows due to a large number of provinces losing connection with the king, and also the huge loss of income from trading (those huge stacks of heavy infantry aren't cheap).