Because AD&D ability checks were based on rolling lower than your ability score. Latter-day D&D players can't seem to grasp this concept for some reason. If you have the minimum 3 Str, you might suck, but you still have a 15% chance of success (on a 1, 2, or 3). If you have a 18 Str, you might be a badass, but you still have a 10% chance of failure (on a 19 or 20). It was really a very elegant system (and I think better and faster than D20 DC checks). If you start allowing characters to have higher than 18 in an ability score, it starts to break the system.
To be clear, I'm not defending percentile strength. I don't actually like it, and I think it could be gotten rid of. Percentile strength was only ever intended for the Fighter class to help balance the class against the other warrior classes. It got abused over the years. A better rule would just be to give all Fighters a bonus to their damage adjustment.
As regards the OP, I think the reason there is no AD&D 2E OSR is because its a fairly strong system as originally published. Its well organised and well written, and well balanced in its way. It only starts to break when you layer on all the splat books and optional rules.