The Mac Studio's disappearance is related to the fact that people now want them for the purpose of running local models. Supply and demand. That plus Apple doesn't shift prices for released products, and it essentially became underpriced when large RAM quantities exploded in price. For the price of 512GB of RAM alone you could get an M3 Ultra with 512GB of unified memory in a nice, quiet, and power efficient package. With the RAM you still need to spend a few thousand more on CPU/GPU, power supplies, storage and case.

Also the fact that an M5 version will be coming, and they likely know they are going to sell out on day one (I expect we'll see a price correction from Apple for higher end configs of M5 studios, base price will probably stay the same), so they need to build up stock reserves.