That's because D&D has always emphasized one path to being good at something. Let's say you want to be the master of, oh, I don't know, climbing. You go "aha, obviously Dexterity is the most important thing here!"
But if your DM happens to be a free climber, and they say "well actually, you need Strength to be a good climber", you're going to feel put out.
And if, over the course of a game, you're asked to make Intelligence + Athletics to plan out a climb, Wisdom + Athletics to find handholds or jumps you can make, Strength + Athletics to make leaps of faith, etc. etc., you're going to question if your character is really all that good at climbing in the first place!
Because D&D is a game that rewards specialization over generalization, and you simply can't have all your ability scores be good. Something will pretty much always be inferior.
Now, in the long run, proficiency matters more than ability, but in most levels of play, proficiency is a small bonus, and you get more from your talent (ie, ability scores) than you do from your experience.