I see your point. But the skills are broken up to reflect various types of movement. The rule that the DM or player can request an alternative ability (preferably the greater one) to use for the skill involved levels things out. If you combine the skills, then why not combine religion and history? After all, they completely go together, especially in the Forgotten Realms setting. Why not combine animal handling and nature? Investigation and perception?
The character growth part of the game means using resources where you want your character to be good. If you want them to be great at acrobatics and athletics, then spend the appropriate resources.
You do realize that while you start out sounding reasonable and forthcoming, you end your reply ignoring everything I just said, retreating back to the unyielding position of "just get both Strength and Dexterity, and get both Athletics and Acrobatics"?
I have repeatedly answered your question "why do this but not do that?"
It's because its unreasonable to ask a character to invest in an otherwise dumpable stat just to get relatively minor movement-related abilities.
It's unreasonable to ask a Rogue to invest in Strength.
Investing in Strength when that gives you an attack bonus and a damage bonus is completely reasonable. Yes, people consider Strength being a bit lacklustre, but still - entirely reasonable. As a bonus, you gain secondary stuff like grappling, climbing, bending bars and lifting gates, and jumping abilities.
But a Rogue already has an attack bonus and a damage bonus. You're asking this character build to invest in Strength solely to complement their movement abilities with jumping and climbing specifically. The rogue doesn't care for grappling or lifting gates, the rogue just doesn't want their great movement to be stopped cold by a wall or chasm.
Everything else you need strength for in real life the Rogue has already been given. Speed, movement, acrobatics, deadly attacks, defense, dodging and weaving, ...
Asking a Rogue to get Athletics as a skill is in comparison a much smaller and less unreasonable ask.
If Athletics can be used with Dexterity.
To sum up:
It's a flaw or gap in how the game rules are constructed that the swashbuckler needs a second ability to go from 90% rogue/robin hood/swashbuckler to 100% swashbuckler. But this is easily patched, simply by allowing characters to use Athletics or Acrobatics for all movement related actions. Or, at the very least, allow characters to use their choice of Strength and Dexterity for both skills.
What's harder to fix is when someone thinks it's a reasonable ask to waste points in an ability you otherwise don't need just to be able to move in the vertical, when you're already been given supernatural speed and grace moving everywhere else... :-/