In British succession laws, if the heir apparent dies before the Monarch, why does the line of succession continue through their children, rather than passing to the surviving children of the monarch?

It's not just the British system of succession but rather all European systems of royal succession. In the expected order of things, the child would be expected to succeed eventually and they are the genealogical representatives of their royal parent, rather than that parent's siblings. Just as one can inherit the crown through a parent, the heir of the heir inherit's his parent's relative place in succession if his parent predeceases the monarch.