It has little to do with YT persons such as Dodoid. Rather, it has far more to do with the combined effects of general lack of supply (re weblacky's comments) and continued commercial demand for SGIs within a number of industries that still use them, most of which are beyond the awareness of typical hobbyists. Calling SGIs vintage doesn't automatically mean they must be cheap or readily available, especially specific parts or systems.
The reality is, any hobbyist looking for certain kinds of SGI item are competing with companies that still use them, many of which operate under extreme cost pressures as regards consideration of replacements, especially in the medical, textile, PCB manufacturing and defense industries. Such companies will pay a great deal of money for the items they want, inparticular the PCB industry which is the prime driver of persistent demand for Fuels and related parts.
An item is only ever worth what someone is willing to pay. SGis are unique in this regard, as they persist in commercial use extensively, but in sectors that most hobbyists know little about. There's no such thing as an inflated price, there's just supply & demand in a free market. 5 to 10 years ago there was a lot of SGI dumping by sectors that could sensibly move on, ie. where performance was paramount (general 3D, CAD, video, animation, rendering, games design, etc.), but these were same fields that had high public exposure through SGI's marketing and popular culture (movies, vfx, games, etc.)
Thus, hobbyists tend to view the SGI usage spectrum through a very narrow lense, missing out what actually matters the most in the current market. You want a V12, DCD or DM6 for Fuel? Good luck, because you'll be competing with half a dozen PCB manufacturers around the world who all want them aswell; I get asked about these regurlarly, especially from factories in Israel, but also the US, Singapore, South Korea and elsewhere.
Old big iron SGIs are also still in use, mainly for vis sim, defense and air traffic control, the deskside Onyx being the most common.
Btw, textile companies are still using systems as old as 12MHz Personal IRIS and R3K Indigo, mainly in the US.