Why Are Used Technics 1200/1210 MK2 Still So Expensive Compared to GR/GR2?

In my opinion, the G series turntables are far superior to the non-G older turntables, so there is no earthly reason why they should sell at a premium compared to a G series, even assuming what you say is true.

The demand for a clean 1200 MK2 is still high driving up the prices. They are so robust they will run for a long time. Plus KAB offers so many upgrades for the new and old 1200 series, you can really improve the sonics for not a lot of coin. The big advantage of the GR2 over the earlier mk2 models is better speed control. (don't forget the stylus riding the grooves is constantly changing the torque required to maintain constant platter speed). If you can stretch a bit, the newer 1300G model is a nice step up from the GR2. Regardless of which you buy you should consider getting the KAB arm dampener and arm resonance cap. 

IF I ever got a Technics, I would add a wood surround to it. These fit the 120, 1200, 1210,

Do they also fit, or are there any that fit the G, GR, 1300 models?

https://forum.audiogon.com/discussions/wood-surround-for-audio-technica-at120-technics-1200-and-technics-1210

Called a Plinth. 

The G series have an entirely new coreless motor with better electronics to control it, a better damped platter, and a stiffer chassis, compared to the earlier SL1200 based TTs. The tonearms are also probably upgraded. KAB is a great company that offers worthwhile accessories, but such bits will not bridge the gap between the older TT s and the G series. However if the criterion is whether you can buy a wood surround for your Technics, then all bets are off.