3e explained the math too. didn't stop everyone from realzing how broken and unbalanced that was after a few years
Actually the core of 3e was not that imbalanced. A few spells here or there became unbalanced because 3.0 dropped caster restrictions.
It was 3.5 and splat books that heavily unbalanced 3e and it was a lack of explanation of the math and the intend behind it what had it break down.
There were a few misconceptions that were later sold as features instead of fixing the bugs:
1. Iterative attacks:
If you accept that tge fighter hits with their first attacks nearly automatically, then secondary and tertary attacks are not loughably useless.
2. Cross class skills:
If you keep to the baseline (DC 10 to 25) and use take 10 and take 20 as explained by the rules, characters are actually very competent with very low investment in skills.
3. Magic items like candy
If you accept point 1 and point 2, magic items are not as necessary as they turned out to be. You don't need AC 30, if you accept that you are hit by fighter's first attacks.
Why did it not work out that way?
1. Poison: if you are hit, you might be dead very soon. Lack of easy healing.
2. Some classes had the need to maxize skills and class skills could be pumped sonhigh, that investing in cross class skills felt like a waste.
3. Poison and spells that disable you instead of just damageing you. You could not afford to be hit. Also, due to the removal of interrupting spells, ll your had were too low saving throws.