Why are these considered options on an equity or equity index rather than options on a futures contract?
They are an Options on a future traded in a securities account just like SPX. I agree, a little odd, but the CBOE created these and wanted the revenues.
The CFE uses the OCC to clear VX futures. https://www.theocc.com/clearance-and-settlement/clearing/futures-products. The OCC also clears all equity, index and cash settled index options traded in securities accounts that are traded in the USA. The CME and ICE clear their own traded products.
Like a sleek and deadly submarine coming up to periscope depth to sight in, fire one - and only one - torpedo, and sink the enemy... and then retreat to the depths in silence, with yet another ship's outline to be painted on the sail. Fear the Reaper.