Obi-Wan says it this way for several reasons: primarily, he and Anakin are at the end of their long relationship and this is one final attempt at reason, but it also is a specific response to a specific extreme statement from Anakin.
Recall their exchange:
Anakin: “If you’re not with me, then you’re my enemy.”
Obi-Wan: “Only a Sith deals in absolutes.”
Anakin’s statement is an absolute statement, betrays Anakin’s Jedi training, betrays the long relationship of mentoring and friendship established by Obi-Wan with Anakin, and serves as the symbolic completion of Anakin’s fall to the Dark Side of the Force and becoming a Sith. All of that aside, at this point, the two are still talking, and there remains a slight chance for Obi-Wan to wake Anakin to the wrongness of his beliefs and actions.
Obi-Wan’s reply, put this way, “Only a Sith deals in absolutes,” then is intended to convey several things:
First, that Anakin’s comment was an absolute statement, one antithetical to Jedi Way.
Next, that it was absolutely wrong with regard to Anakin’s assessment of Obi-Wan and Anakin’s relationship; Obi-Wan can clearly love Anakin and remain his friend while disagreeing that siding with Palpatine and murdering Jedi younglings is a correct action to take.
Finally, Obi-Wan is intending to convey he knows Anakin is a Sith, and hopes Anakin realizes this.
Obi-Wan’s statement is not intended to theorize about a universal law; he is aware there might have been or may be Sith who deal in things other than absolutes. Rather, he replied in this way to try to save Anakin from further falling to the Sith ways.