It is technically possible as a 3.5" HDD will require 12V @ around 30W, but while USB C Power Delivery full profile support is on the Macbook for power the laptop itself, it isn't clear how much of USB C Power Delivery is supported bus powering other devices. That would also spread to other USB C enabled laptops.
Since enclosures and cables aren't selling for top dollar like they used to I'd imagine there just isn't room in the market for a "premium" USB 3.5" HDD enclosure or cable so manufacturers are waiting for USB C power delivery profiles for bus powering being a bit more universal and/or clearly explained to users.