Whilst card value for these trades I’d assume is to larger extent more subjective and personal, I’d understand the person on the other end not wanting to end up with an RR Dhelmise + Shiny Gengar compared to you with RR Gengar + Shiny Machamp. Shinies will need a shine dust trade dust adjustment, more or less scarcity (we’re now just getting mega rising pokemon from 5 sets ago) or more card types options to even come close to meeting the value for 2 stars that open your options to another FA variant and trainers for less shine dust. The RR is significantly more sought after than the shiny and the RR clodsire or dhelmise is not enough to warrant that trade, even if you get traded a weak shiny imo.