If they belive he is a god, doesn't he bevcome a god? I was of the understanding that that was how it worked.
Try worshiping gravity, you'll get a similar response from it (nothing). No matter how much fruit, people, and animals you throw off a cliff to appease gravity, you'll still fall at an acceleration of 9.81 m/s^2 like everyone else. Same deal with Sithis. And the first few paragraphs of the Monomyth should help.
All Tamrielic religions begin the same. Man or mer, things begin with the dualism of Anu and His Other. These twin forces go by many names: Anu-Padomay, Anuiel-Sithis, Ak-El, Satak-Akel, Is-Is Not. Anuiel is the Everlasting Ineffable Light, Sithis is the Corrupting Inexpressible Action. In the middle is the Gray Maybe ('Nirn' in the Ehlnofex).
In most cultures, Anuiel is honored for his part of the interplay that creates the world, but Sithis is held in highest esteem because he's the one that causes the reaction. Sithis is thus the Original Creator, an entity who intrinsically causes change without design. Even the hist acknowledge this being.
Anuiel is also perceived of as Order, opposed to the Sithis-Chaos. Perhaps it is easier for mortals to envision change than perfect stasis, for often Anuiel is relegated to the mythic background of Sithis' fancies. In Yokudan folk-tales, which are among the most vivid in the world, Satak is only referred to a handful of times, as "the Hum"; he is a force so prevalent as to be not really there at all.
In any case, from these two beings spring the et'Ada, or Original Spirits. To humans these et'Ada are the Gods and Demons; to the Aldmer, the Aedra/Daedra, or the 'Ancestors'. All of the Tamrielic pantheons fill their rosters from these et'Ada, though divine membership often differs from culture to culture. Like Anu and Padomay, though, every one of these pantheons contains the archetypes of the Dragon God and the Missing God.
Also 'a lot of people' is BS. The Hist only take note that it was Sithis that was the one to cause the first reaction. Even the Redguards call it "The Hum," as it's just an ever present force. Hell, the only group that seems to worship Sithis is the Cyrodiilic chapter of the Dark Brotherhood. That's not a lot of people.