[[There are no TBS games where the AI is all that smart.]]
Obviously you haven't played ALL TBS games then because there are sevearl TBS games with AI's that can whip the socks off of you.
"War of the Lance" for instance is one of the better games of the 80's with an AI that is downright ruthless and hard to beat.
"Spartan" v1.013 has one of the best AI's I've ever played against and another that will whip the socks off of most players on just the "normal" setting (it was so hard they had to DUMB IT DOWN in patch 1.017 because of all the whinners saying they couldn't beat it). Move on up to "impossible or very hard" and you will see smart and ruthless and grueling play vs the human player.
Panzer General has an excellent AI in campaign mode another that will beat the soxs off of you.
Civilization IV has an excellent AI and will beat the soxs off of you.
Combat Mission with certain settings will beat the soxs off of you.
TBS AI can be smart if players will give them their smarts (that have been programmed in) and up the difficulty levels. Normal is for unexperienced players. Very hard and Impossible are for the brains and those with IQ's over 100.

And the AI show's it's smarts when you play the extreme difficulties.

No, it's not cheating, it's just smarter, people just tend to call things cheats when the AI beats them over and over and over again. lol But, of course humans never cheat the AI right? Humans never exploit a mechanic of the game to beat the AI right? And when a human does this he/she feels "smart" correct? So, I see no cheating going on by the AI when it exploits something vs the human like advantages in what it sees and techs it might get at the start of the game or speed of teching up, etc. etc. They humans cheat, the AI should have the same advantages and cheat too. That's SMART!!!