Here is an example of one attack from pathfinder 1e, from a previous post:
So this is a pathfinder example, but I know that it would be near identical in 3.5. My character wants to attack a foe. Should be simple right?
Well, my character is an alchemist, so his BAB is +5, +2 for dex, +1 for the train firebug feat and +1 for the feat throw anything, for a total of +9, and it's vs touch AC. This is a slightly complicated calculation, but you only have to do it once per level, so it's good right? weeeeeeelllllll
I have drunk the mutagen, which gives me +4 to dex, which means +2 to hit. I've also cast reduce, which increases my attack by 1 and gives me 2 more dex so another +1. The foe is 25 feet away so point-blank shot kicks in, giving me another +1, BUT there is a -2 range penalty. The bard is signing that's +5 right? (our bards is *awesome) - nope the bard is more than 30 feet away from me, reducing the bonus to +3. I'm also firing into melee (-4) and there is some cover (-2) so that's not great... but wait I'm hasted by the sorcerer, so I get another +1! So now it 9 +4 +1 +1 +1 -2 +3 -4 -2 + 1= +12 (... I think).
And this will change every round - did I take just take dex damage, or been hit by a debuff? did range changes, is cover less (or more), did a buffing spell expire, the bard stopped signing, etc etc etc.
(this is not a fictional example. This is how our game goes, and this is my character).
I roll poorly - a 3 - and I hit touch AC 15. Does this hit, I ask the GM? Easy question right?
But no, we're not done - the monster's AC may be changing every round!!! The monster may have cast some protective spells - which may or may not apply, and and may or may not have been dispelled by the party. Furthermore, other players may have put a number of debuffs on the foe, some which stack and some which do not.
All this work for a single attack...
Edit: I'm not saying that this is "wrong". Some people enjoy this kind of crunch and mental gymnastics. But as I grow older, it's not so much fun anymore. I can handle it just fine, but it slows the game down, combat takes much more time than other games (you should see troika!) and it's difficult for a number of players. I didn't know until a few years ago how difficult for some people this kind of math is."
I've been accused of "exaggerating" but no, this is from a real session, and this happens frequently. If this seems awful to you, stay away from PF and 3.5! On the other hand, if you thought it was fascinating... it may be the game for you
If you stick to the core rules it takes a while until you get to that point.