Because it’s MTB kit. Nuff said.
When it comes to fluids and parts, a company will just take standard kit and increase the price. Let’t take an anaology:
two identical shirts, made in the same factory by the same people. One has an “Armarni” label sewn on it, the other has “Primark” sewn on it. By virtue of the Armarni label, the product is suddenly worth signficantly more, because people pay for it.
Brake fluid and suspension oil is exactly the same, but “Mountain Bike” becomes the Armarni label and suddenly it’s worth twice as much. I get my oil from a MX shop. It’s £5 for the litre, half what the same volume in Stendec oil is and it’s CHEMICALLY IDENTICAL. Same with fox stuff.
Brake fluid is exactly the same, what’s in a Finish Line bottle is chemically identical to a bottle of Halfords DOT 5.1 but twice the price.
So yes, Oil is Oil. Get some fully synthetic Suspension fluid in the correct weight from a Motorbike shop and it’ll do the job just as well as the Fox. I ride DH all the time and I found absolutely NO difference between HFX stuff you get for motorbikes and Stendec oil other than price.
EDIT: TJ is right, despite being chemically identical it voids your warranty, but that’s your own fault for buying Fox! 😉