Rugby Union

What I want to know is, how do teams like Australia and New Zealand consistently manage to field good sides, when the likes of England and Ireland go through huge ups and downs..? Surely it isn’t a population thing - New Zealand can’t be much bigger than England, if at all - so what is it?

The same thing happens in cricket - Australia are always good. When we are better than them, it only ever lasts a year or two! Why can’t we manage to compete (I mean really compete, with a genuine chance of winning) at the highest level?