ive got a question why do the AI spearman units......and only the spearman units have the magical ability of kicking the butt of even a ARMOURED TANK!
its insane...
you'd expect the spearmans brains to be splattered over the wall of the city but noooooooo its the tank that exploded due to tiny bronze spear wounds

im just asking why does this happen?
Does Sid Meier have a fondness for spearman and felt that they should be invincible?
"spearman" is just a fancy label the creators of Civ3 have put on a construct that contains the values 1/2/1. These variables influence the outcome of a
chance based function.
The function works such, that the chance a units with a defensive value of 2 will win rarely against a unit with an offensive value of 24. While the chance is not zero, it is still very small.
They could have called the spearman an "urapit" and the Modern armor an "rodalot" and put it in a whole dreamed up setting. If this where so, you would not have complained about the small chance the "urapit" had of winning from the "rodalot." Even though you'd technically be playing the exact same game, except that the units had been given different labels, and maybe even different graphics.
As for "realism" I think it is quite realistic, as it is not impossible for a group of badly armed people to disable a tank in real life. Its difficult, but with some creativity, it can be done.
And it is also an important part of gameplay. Its a strategy game after all, If losing an odd tank to a spearman once in a blue moon is setting you back, then there was obviously something wrong with your strategy.
If this answer doesn't satisfy you, then you are probably happy to hear they improved this in Civ4.
In civ4, they made it so that a single spearman can not win against a single healthy tank. But the spearman does have a chance of damaging the tank. And once damaged enough, the tank can then potentially be defeated by an other spearman. Much more realistic!
Making it so a tank
always wins vs spearman, would be less realistic...