AI of most turn based games sucks and is easily defeat-able by humans.
Same with most bots from FPS
is it because they invest about no time making their AI?
FPS AI is mostly about impairing the AI after coding, not trying to have the AI kick ass (it's made to mimic a brain-dead player). Most games have you competing against 5-20 AI units at a time, so you'd lose the badass feeling devs strive for if AI were competent. If FPS AI was difficult, multiplayer FPS services wouldn't spend so much in resources trying to get aimbots off their servers.
RTS AI is waaaay more complicated because it has to think about the future of its actions, what actions it can take mid-term, then long-term, and see what he knows about the human's strategy to counter-act his plays. Those lead to the "branching computing crawl" - for every action it considers, there are probably 2,000-20,000 moves it can make after that, then another 2,000-20,000 after that, and after, and after, and after, until it has its "best final build strategy" all figured out. Because of this, it usually only has a few preconfigured strategies it randomly selects and then makes minor adjustments based on what it sees the human and other AI doing. A dev company with loads of resources, however, may take the time to introduce a bunch of off-the-wall sub-strategies to AI's arsenal. Humans are very much the same, as far as how they think, but learn to beat the AI by having a larger set of "preconfigured strategies" -- basically, humans are much more random. AFAIK, there aren't really self-learning games, where it'll do something like have the AI watch a match it lost, then try a bunch of different strategies and sub-strategies to win, then save that strategy if it sees the player later trying to pull the same shit. I'll say that custom AI for Starcraft 2 released soon after its release was pretty difficult to beat.