Sam2: You are correct that a siddur has Shem HaShem, but it doesn’t have the Kedusha of a Sefer Torah. Would anyone think of laining/getting aliyahs (with a Bracha) from a printed Chumash (it also has Shem HaShem)?
There is even discussion about our ‘leniency’ of using printed Tanachs, Chumashim, etc… for Kriat HaHaftorah when in fact we should really be using Klafs (like Megilla) [whenever financially feasible].