What KA is doing, and what others are doing is different. Although it's not clear, KA seem to want to feed their ego by having 10 KA alliances at the top. By beating the same alliances from below, they prevent these teams from ever reaching the top.
Other alliances leaving, is for multiple reasons. Some alliances break up. Some are tired of playing for 5 glory points. Some are tired of being targeted for their points by alliances using p2w. In any case, if people leave their alliance in the top100, they should close it, or give it away....something we have done 3 times now.