Because there will always be parts that eventually fail. It's important that those parts be repairable or replaceable because you can't count on the whole thing lasting forever. Nothing does.

I still fret over the lifetime of my belongings. When I hear a mouse is tested and guaranteed to function up to several million clicks, I think of that like an approaching deadline. In reality, that many clicks would take decades on average. Ideally that's the kind of lifespan we're talking about when we purchase a product. Personally, I've never had an OLED phone burn out to the point of unusability or even minor annoyance after years of heavy use, more use than my Steam Deck will ever likely see. But if/when it does happen, it's good to know that I could replace that component.