When discussing which beliefs are heretical to orthodox Judaism the phrase "there is no shulchan aruch on deos-beliefs" comes up often. I would like to know why this is the case. Why can there not be a single set of canonized, agreed upon beliefs which if you don't hold, l'halacha you are a kofer/apikorus/min?
I think the premise of this question is mistaken, for a few reasons:
The Rambam's 13 principles of faith are fundamental beliefs. They are said to be based on the 10th perek in Sanhedrin which lists things that "אין להם חלק לעולם הבא" e.g. non-believers of resurrection of the dead, non-believers of Torah from heaven and apikorus. The remainder of the perek lists more.
The Rambam repeatedly wrote that we can't make conclusive/authoritative on matters of faith/hashkafa:
"And I've already written several times, that when there's a dispute between the sages in arguments of faith, the purpose/end is not to [establish] practice- since we don't say the halacha is like so-and-so and against Ben-Azai" (Commentary on the Mishnah, Sotah 3:5)
"All disputes amongst the sages that have no practical application, but are only in matters of faith, there's no position to decide halacha according to one of them" (Commentary on the Mishnah, Sanhedrin, Perek Chelek)
(Commentary on the Mishnah, Shavuot 1:4)
(ספר המצות, לא תעשה קל"ג)
(מאמר תחיית המתים)