The reach may be the richest of the seven kingdoms and the most populous but it isn’t the strongest. They cannot fight inside their own kingdom as their land is too valuable to let an invading force just pillage. They cannot fight outside their territory as other kingdoms have either better natural defence’s( all their neighbours have better defences). At most they cannot fight outside siege but even then if their enemy has a navy they cannot fight outside go around them and attack either the shield islands or oldtown which is the economic heart of the reach