Because they're designing a game, with at least one-and-a-half eyes focused on the game play.
Many versions of D&D bundle expertise in archery and expertise in melee fighting together.
The DEX stat bundles speed (of movement), speed (of reflexes), deftness etc together, so that someone who is good at (say) cartwheels is also good at (say) parlour magic.
The WIS stat means that those who are intuitive in their thinking are also good at spotting ninjas. The INT stat, in 5e D&D, merges Sherlock Holmes and Marie Curie.
Again, this is all with an eye to game play. If, as a GM, you want a NPC who can climb well but is at risk of drowning if they fall from a boat, then you can just write that NPC up that way.