According to the gemara (Megilla 29b, S"A O"C 685) there is a mitzvah to read the special reading of Parshat Zachor on a particular Shabbat (before Purim). This mitzvah is listed as one of the 613. I have been studying the sources and the discussion and am left with many questions, including "why is there no bracha on this mitzvah?" and "is the essence the remembrance or the (particular) reading from the torah?" but I'll skip that for this question:
The gemara discusses in Megillah 18a that remembrance has to be performed through reading from a scroll (like a megillah or a torah) -- not by heart and not from anything else. In fact, this commanded remembrance is said to be the only min hatirah mandated torah reading! But there are 6 zechirot that we have in our siddurim including 3 which use the same imperative language of "zachor" as is used to tell us to read Parshet Amalek.
If they are all zechirot, and the gemara says that zecher is performed through reading from a scroll, why are we not required to read the other sections of the torah at least once a year to be mekayem those mitzvot mide'orayta on the same level of magnitude as the zechira of Amalek?