The Gemara concludes that blessing is a "sevara" that one is forbidden to enjoy this world without blessing, why then all blessings (besides Birkat Hamazon) are Derabanan (as proved from "Baal Keri")? "Sevarot" have the severity of Deorayta as the Tora is not telling us what we can achieve by ourselves.
Thanks in advance.
Your question is excellent. The P'NEI YEHOSHUA asks it and leaves it unresolved.
The EINAYIM LA'MISHPAT answers that there are different types of Sevaros, some d'Rabanan and some d'Oraisa.
RAV GUSTMAN zt'l explained that even though a Sevara has the severity of a d'Oraisa, nevertheless it is not able to obligate the observance of a new Mitzvah, but only to modify or clarify an already-existing Mitzvah.