You are correct, of course, that nothing is produced in the world without the Hashgachah of Hash-m. However, when we acknowledge our appreciation to Hash-m for what He provided us with, we must thank Him for what we naturally view as having come from Hash-m. A man who toiled many long hours and put much effort into pressing olives in order to obtain the oil -- although he realizes that the oil comes from Hash-m -- knows that he was the one who put in much sweat and toil to produce the oil. If he were to thank Hash-m for the oil itself, he might begin to think that he is thanking himself and not Hash-m. In contrast, though, when a man plants seeds and then, miraculously, an entire tree forms and then olives begin to grow, that is naturally viewed as coming from Hash-m.

Moreover, earlier in the Gemara in Berachos, the Gemara cites the verse that says "v'ha'Aretz Nasan li'Vnei Adam." Hash-m gave the land to man to work and to produce. In man's Hanhagah in Olam ha'Zeh, he is entitled to recognize that Hash-m gives him things when he makes a Hishtadlus for those items (notice that if a man sits in his easy chair, without doing any Hishtadlus, and waits for olive oil to appear on his table, he will wait a very, very long time, and no olive oil will appear). When man exerts Hishtadlus, then (if he is worthy, and sometimes, even when he isn't), Hash-m gives him the olive oil. But Hash-m gave him the olive oil only as a result of man's Hishtadlus, and therefore it is not appropriate to enact the Nusach of the Berachah to that effect, because Hash-m wants man to know that his Hishtadlus is important, and because Hash-m knows that man would not be able to properly thank Hash-m for something into which he put so much effort. (See also Medrash Tanchuma, Parshas Tazria 5.)

Rabbi Y. Shaw