Why? Because that gives a framework to the immersion/pixel bitching divide that the DM can learn to be consistent within. You don't have to ask to look behind the statute because the DM is being a dick. You have to ask because your skill is lousy, but asking in a particular spot reduces the challenge by a rank, allowing you to then roll. Succeed, and you get reasonable stuff. Fail and you get nothing.
I think this is part of the divide in itself. The idea of numbers vrs actions.
I guess the question really is, does the existence of numbers in and of itself cause people to forget to even bother to announce what they're doing?
Earlier forms of D&D caused you to interact with the environment by default because there wasn't much you could do otherwise. (And if your DM was a stickler you really DID have to get into detail about exactly how you were interacting with the environment...) I don't think this is a "better" design, but did it accidentally lead to more immersion?
Not only did you have to describe say the bed that could be searched, but since the players ability to do anything relied solely on what they knew about the bed, you needed to describe it in detail so they could formulate a plan. After all, how would they know they might have to search UNDER the bed if they didn't know it wasn't the type that has sides that go all the way to the floor?
With skill ranks the DM can just say There is a bed in the room, and the players can say I search the bed.
I don't think this implies skill points or ranks are entirely to blame in and of themselves though. I think it's just as they stand, there isn't enough incentive to cause you to interact.
Nothing in the rules changes the DC if you say you're looking under the bed, as opposed to just saying you search the bed. By the rules using whatever skill you use implies you are doing everything necessary to complete the task.
I think as others have pointed out this is a direct result of people spending countless hours describing every facet of their action. I search under the bed. I search under the bed for cracks. I search under the bed for bumps. I search under the bed for discolorations, etc...
I would say this can be seen in the idea of taking 10 or taking 20. Instead of saying all those things taking 20 or 10 implies I do all that stuff.
I think in the end we don't need to dial things all the way back to pure DM fiat, but instead we need to find some compromise between the two. A system that allows your character to be better then yourself on paper, but at the same time promotes a bit of interaction with the imaginary world.
How do you balance those two things though?