The designers have designated certain aspects of the game to become virtually automatic in the vast majority of adventuring scenarios once a character specializing in that specific skill reachs around 10th level. Casting Defensively happens to be one of those aspects. Of the top of my head, the list would include, but is not limited to, Spot, Listen, Hide, Move Silently, Diplomacy, Tumble, Knowledge, Bluff, Survival, Search, Disable Device, Open Locks, and Ride checks.
Now it is true that even a eagle eyed Elf Ranger PC may occasional match up against an NPC Halfling Rogue. But the fact remains that any character that can hit a DC of 25+ the majority of the time is performing at the limits of our experience of human competence all the time. All but the most rare NPCs are helpless to directly oppose the PC in his area of expertise at those competence levels.
Eventually mages do not have to roll the die to successfully Cast Defensively. It is also true that eventually the knight in shining armor can automatically Fast Mount and Fast Dismount (Ride), and the Ranger can follow unerringly any fresh tracks not obscured by magic. In the context of the game, this simulates allowing your PCs to do heroic things through explicit competence. A 5th level character is not at the point of automatic success, but succeeding the majority of the time is not a problem but a mark of the transition period towards eventual excellence.