Players don't play games for mechanics. Ask a player what they like about a game, and they usually describe driving in a getaway, slicing opponents to smithereens, or watching an enemy flip end-over-end through the air. Ask them what they look forward to in a game they want, and they'll talk about being a badass, or being afraid, or exploring a new environment. A few people might describe loving a game because of how its inventory system blended with advancement, but even designers that sometimes play games purely for mechanics more often than not go into games for the sum experience. Players don't love games for their art, technology, audio, or design, alone. What they seek out and remember is a sum experience.
So why are games so often built mechanics-first? How often does a designer choose gameplay x as the basis of development, asking the story team to come up with a wrapper for it, and the artists to create art that highlights it, and programmers to build technology around it? Why is x the core when players care about the experience? Gameplay is a critical component, but just like audio, art, and programming, design (regarding gameplay mechanics) is no more important than other disciplines in their potential to influence an experience. Sometimes, it may even be less important. More on this tomorrow.