I feel that the game is likely designed for the multitudes of players who are not top quality competitive players. For most people, the artificial intelligence works exactly as it is intended to and provides a rewarding game play experience. It provides a challenge but one that can be overcome. The best of the best players will never be satisfied and make up only a tiny fraction of Firaxis' consumers, so unfortunately they cannot spend all their efforts to please the best players. They have to think first of how the game can be created to make the largest number of people happy with it. They do help the great players though by offering the tools so that they can change the game to how they want!
I find it offensive that someone said "a stupid dog could beat this game". That is absolutely not true, and there are thousands of people who struggle to win even on the easiest difficulties. Those people are not less than "stupid dogs". I know it was meant as a hyperbole but it demonstrates a lack of consideration and a dehumanization of players who are not as skilled as the poster.
The computer players successfully build empires, travel through the technology and civics trees, try to achieve victory, and can cause stress for the player. Please remember that for most people this is perfect! If it is not enough of a challenge, there are options such as playing against equally skilled human opponents or tweaking the game for yourself to be the way you want.