Why is CVD not used in the manufacture of NMOS gate oxides?
CVD is used for modern gate dielectrics.
The simple answer is it is much easier to thermally oxidize silicon to get a high quality gate dielectric than it is to deposit one. Therefore, you use oxidization of silicon for gate diectrics whenever you can. Additionally, most CVD techniques deposit too quickly to precisely control the handful-of-nm thick gate dielectrics used in modern CMOS. But, current state of the art CMOS does in fact use CVD gate dielectrics.
CVD (chemical vapor deposition) is a class of thin film deposition methods. Most of which are too fast, produce too low of a material quality, or are otherwise difficult to precisely control, to use for depositing a gate dielectric. One of the deposition methods under the CVD umbrella is ALD (atomic layer deposition). Using ALD you can precisely control the thickness of a deposited film by depositing a single atomic layer of material at a time. This is used in modern CMOS gate dielectrics, usually to deposit a hafnium based oxide. This allows us to use materials with much higher dielectric constants than you get with silica.