This is because you refuse to acknowledge that without mechanics to back it up, your RP means virtually nothing. Its why changing a Fireball from a ball of fire to fireworks is perfectly fine, cause nothing mechanical about Fireball is changing. In this case, you're attempting to use RP to back up the idea your character is bad at Strength or Dexterity when the mechanics of your idea show the exact opposite. This utterly fails because the mechanics you came up with state you're just as good at Dex checks as you are Str checks, because they have the share the same score. It doesn't matter how much you RP or flavor it, you're still just as good Strength checks as you are Dexterity checks and vice versa.
So, people do want their characters to succeed, but they also want a clear differentiation between the two. A Fighter with 20 Dexterity plays very differently from a Fighter with 20 Strength. One is likely wearing Heavy Armor and charging in with a Greatsword, the other is likely wearing Light Armor and standing in the back firing an arrow. We don't want the Fighter with 20 Dexterity to suddenly pick up a Greatsword and Heavy Armor and be just as effective as the Fighter with 20 Strength. Because at that point there's no real way to differentiate between the two.
And while you can claim "But you can choose not to do that" or "you can RP that you don't do that" all you like, the fact of the matter is that a single physical stat would have that be the norm, and anyone who doesn't carry a Longbow and either a Greatsword or Longsword and Shield will be purposefully weakening themselves for no reason what so ever.
This...doesn't actually happen as often as you might think. It can happen occasionally, but even then, its a very rare case.
See, here's the thing, core strength doesn't actually exist all that well in DnD. Its why you can have a character with 8 Strength, 20 Dexterity, and 16 Constitution be perfectly fine. Or you can have 20 Strength, 8 Dexterity, and 16 Constition and still be perfectly fine. While it may not be a common archtype outside of DnD, whichI am not sure that's true, it is very much a common archtype in DnD.
Actually, a good example from movies for someone who's extremely nimble but lacks strength is Inigo Montoya from Princess Bride. Throughout the movie Inigo is shown to be extremely nimble, highly skilled with a blade, but he never has any great shows of physical strength, like what you'd see from a high strength score. He's pretty good at acrobatics, as shown in his fight with The Dread Pirate Roberts, and has a high Constitution, as shown when he gets stabbed multiple times, but he's never shown to have high physical strength. Meanwhile Fezzik is the exact opposite, extremely strong and capable of great feats of strength, but lacks the dexterity Inigo has.