It's because once you're top dog, you stop aiming high and start maintaining your top spot. You stop being ambitious and innovative, instead you start being anxious and wearing rose colored glasses.

That leads to complacency, corruption, and delusion, ultimately leading to failure.

Everyone and everything from mundane individuals to megacorporations and empires have all fallen from grace once they became top dog. No exceptions.

If you want to last, aim high but don't become top dog.