Why? High rolls are better for both attacks and saves. In 2E the stats decrease as you improve, but that's because the stat generally represents the number you need to roll to do the thing in some specific scenario.
Like THAC0 is the number you need to roll to hit a target with AC 0. You then subtract AC from THAC0 and if you rolled that or higher you hit. For example, if you have a THAC0 of 17 and an opponent with an AC of 7 you need to roll a 17-7=10 or higher to hit.
Same with saves - if your save vs death was 16 and you needed to roll a save vs death, a roll of 16 or higher was a success.