The gemara says in the begining of the 6th perek that we make berachahs on food because of a sevarah. A sevarah is ussually deorisa so wwhy aare berachos only derabonon? They should be deoria like other sevarahss?
yid again, USa
While many commentaries deal with this question, a simple explanation is that of the Tzlach. The Tzlach explains that while a Sevara is a Torah principle, this is applies in places where logic dictates that a certain claim is correct (see Kesuvos 22a regarding "Peh she'Asar" and Bava Kama 46b regarding "ha'Motzi me'Chaveiro"). It is possible that we also use logic to understand the parameters of certain laws that are stated by the Torah. However, we do not find that because something is a logical practice that we make up an entirely new Mitzvah mid'Oraisa. Otherwise, the Torah would not have to say any logical Mitzvos (i.e. Kibud Av v'Eim), and would rely on us to understand that they are Torah laws. Therefore, it was up to the Rabanan to institute that we must say Berachos before we eat.
All the best,
Yaakov Montrose