While it appears you might be onto something, that is not actually the case. Itachi has a personality, just one that was'nt portrayed(except for a few times) by kishi to us readers clearly.
War and suffering and loss shaped him as a child, that was the time he was weak, vulnerable and not in control of the situation at all, contrary to what you state in the OP.
The way we saw him, he became that way after having gone through years of self conflict and turmoil until he was finally able to put up a hard exterior that seemed invincible/perfect. What was underneath all that was the real Itachi. It was quite accurately stated in the 3rd databook about his personality, a caring brother and a kind person, that he would choose to be a ninja was the ultimate irony.
Imagine someone being a naturally gifted sportsman of extreme abilities but who does'nt actually live for sports but kinda just reluctantly carries on playing. Something along those lines, and while no exact real life copies exist, there are similar people in rl. You just have to substitute it like this ' for a shinobi, to do something like that is like a sportsperson to ....'
Now, i am 100% sure that Itachi did/would've lost to some people, maybe when he took the chuunin exams or w.e. The only thing is kishi didnt show it but does'nt mean it did'nt/could'nt have happened.
I think what you are missing, the most crucial point is this:
Itachi knew his limits. So, if there were a fight/situation where he knew he could'nt win, he would'nt get into it or he'd get aid in it. Someone like naruto tries to fight everyone, and can get beat up in the process(he's got guts but no brains lol).
Itachi on the other hand: he knows he may lose to madara, so he leaves that to naruto and bee. He had a good idea that taking on kabuto and ending his edo tensei tech was something he was more suited towards so he opted for that.
Take his time in Akatsuki. He 1 shotted Oro and deidara, but he never even tried to confront Obito or Pain. Because he knew he would lose, and how far he could go. That was the thing about him. So it naturally appeared as if he were invincible, its just that he knew himself and what he could and could'nt do.
Hope that made sense.