There are a few reasons why warring kingdoms struggle to compete in top play.

You can look at divinity this age for instance: they have won all their wars, but still were unable to keep up in growth.

Or balance last age: they had some strong play early, but then were caught off guard a couple of times and lost big.

And when you try and fail, it can often be demoralizing and lead you to stop trying.

As for the reasons:
* You war less at larger sizes (purely a matter of available targets), which is less exciting. Top play involves boring periods that still need significant activity. And though some ages you may war 6 times, other ages you may only war once or twice depending how the age plays out.
* Top play requires full understanding of econ during growth periods. This is where warring kingdoms usually struggle the most. Your typical warring kingdom is used to pumping for a while, then warring someone else who's also pumped for a while. But pumping while also whoring 2x a day requires a different approach, in both build and in your race/pers setup.
* Top play can be very brutal, and diplomacy is essential. Divinity screwed this up by falling behind rage, without ceasefiring them under a long enough term to let them catch up. They ended up getting bottomfed-waved by rage. They knew they couldn't win a war and had to give a free wave.
* Even warring strategy is different. Warring for wins, warring for land, and warring for honor are all different. Top kingdoms are expert in maximizing land gains once the war is won. Overall this is minor compared to the other points, though.