Hilton decides whether it wants to offer commission via "affiliate marketing". The cashback and airline shopping portal websites piggyback on that and offer to return some or all of that commission to users. Other website owners (i.e. blogs, etc.) might keep it for themselves.

Anyhow... Hilton has clearly decided that it's not worth it. Especially for elite status members who they reckon are already inclined to book direct.

Long story short... if you actually did find a cashback website offering Hilton cashback, it's probably a scam and/or the transaction won't actually post properly because Hilton won't actually pay the commission.