The problem with stat bonuses

The only thing that happens when player characters level up in O/AD&D is that their THAC0 decreases and their Hit Die total increases. That means that their chance of hitting an enemy gets better, and they can survive more hits. Their damage per hit does not increase, and neither does their Armor Class, which is their chance of not getting hit. This means that a level 10 character is as vulnerable to a hit as a level 1 character as long as they wear the same armor.

Strength, Dexterity, and Constitution modifiers all change these probabilities. Strength bonuses decrease THAC0 (thus increasing chance to hit) and increase damage done by a flat amount. This means that fighters with melee weapons are effectively attacking at a higher character level. Constitution bonuses increase hp per level, effectively allowing player characters to take hits as a higher level character. Dexterity bonuses are the only way to lower a PCs AC outside of equipment, so it is a character's only innate defense against getting hit by monsters of higher level.

The end result of all this is that it becomes impossible to balance combat challenge around a player party level. The only possible way would be to add up all Strength and Dexterity missile to-hit bonuses to find out what effective level the party is attacking as, and tally up all Constitution and Dexterity to-hit modifiers to determine what effective level the party is defending as, and then craft a unique monster or set of monsters within that narrow band to provide the players with a challenge.

This is why, I guess, OSR groups look down on 'encounter balance', no one wants to do the math and if they did, all encounters would end up looking the same. Once PCs get their hands on magic items, not only do simple +bonuses to hit and damage change the math but magic items with special abilities change the whole manner in which combat is played. This asymmetric gameplay is what I think attracts people to OSR style combat, but it is too easy to make encounters either too easy, or too difficult for the players.