Just to add to earlier comments -
It's an interesting question.
One of the main attributes of the gate is low resistivity (or high conductivity), so, theoretically, the ideal name for the structure would be conductor - oxide - semiconductor. But, in early days, metals were used as a gate material - that's why the name - MOS (metal - oxide - semiconductor), and it stayed this way, at least for historical reasons.
"Poly", on the other hand, has many different "options" (so to speak) - low-resistivity, high-resistivity, silicided, non-silicided, etc. - so, a term "poly" itself is not associated with low resistance, required for gate material.
By the way, in recent technologies, companies returned to using metal gates, instead of polysilicon gates (mainly to avoid polysilicon depletion effect).
Term "oxide" is equally rather conventional - as other dielectrics can be used instead of silicon dioxide, and, in recent technologies, complex dielectric stacks, with combinations of different materials, are being used.