It all depends on what you read. Now, I know that to U.S. people this will sound classist and snotty and snobbish, but there is a lot of undiluted crap in libraries and bookshops.

Even if I would very much like to, I will not inflict my reading tastes on y'all - but I agree that it is certainly a very stupid thing to praise reading in itself. On the other hand, praising reading of "good stuff" would lead instantly to political incorrectness, because then you would have to define what the good stuff is: and out comes the Western Canon, Dead White Males, crash * boom * political death. Hence the very bland "books are good for you" campaigns.

On the other hand, I don't buy the argument that the natural environment is such a wonderful thing. First of all, nature does not give a damn whether we are alive and happy or carrion. Secondly, what distances us from our natural environment is a lot of what makes us humans: art, trade and, generally speaking, civilization. Lastly, what would be man's natural environment ? Living butt-naked in the veldt, being occasionally eaten by jaguars and waiting for the big black monolith ? Or something more idyllic ?

True life experience is always mediated by your previous experiences and prejudices: the only viewpoint without prejudice, said Ortega y Gasset about art criticism, is the orangutan's. Those prejudices will get into your head no matter what, because from our earliest years our parents, our neighbours, our friends tell us things and opinions. Some of them we make our own. Reading lets us hear (although through a narrow band medium) different voices from faraway lands and times.
Will a walk in the woods expose you to the viewpoint of a Nigerian ? Of an Italian from the Renaissance times ? Of a Russian anarchist ? Will you meet Leon Trotzky in those lovely National Forests ? Or will it be just more people like you ?
Don't get me wrong here, I like walking in the woods (and hiking and camping ...) and to me it is wonderfully relaxing and pleasant. But, to me it is escapism (a benign kind of). It abstracts me from my fellow men, which is a very good thing for a while. But I understand nothing more: Mother Nature is quite dumb, particularly for someone not given to sudden Pantheistic moments of extasis.

In this sense, there may be some sense in the generic lauding of reading: since books (and generally speaking, printed material) is cheaper to make than television productions, there is a better chance that it will expose the reader to something that may, for lack of a better expression, broaden his mind.

By the way, the idea that contact with nature improves the human being is (surprise, surprise !) a literary one itself: Ralph Waldo Emerson, and the Romanticism movement are largely responsible for it.