I think (I do not KNOW this for certain.. I am definitely not an expert on western hunts) it has a lot to do with carrying additional weight and bulk that most western hunters dont find necessary...
The guys Ive talked to that do a lot of hunting on foot in Idaho and Montana have told me that they absolutely use a rest.. especially when shots are long... but that rest is typically their pack... or they make an improvised rest/improvised sticks using their trekking poles, or they use a convienient tree branch, etc..etc..
Theyre walking as much as 10 miles a day (sometimes more), already carrying a 20+ lb load for just day hikes (can be a much heavier pack if theyre actually living out of the bag as opposed to deploying in and out of a camp somewhere each day), often at elevations ranging from 5500-10,000 feet, etc.. so every ounce matters.. as does every available cubic inch of the pack.. and since they have other adequate rest options available (already carrying trekking poles for example), its not worth trying to factor in another couple of pounds and a location on the pack to strap sticks down to...
The above advise was one of the primary reasons I selected a Mystery Ranch Pop Up pack for my upcoming idaho hunt.. Mystery Ranch pretty brilliantly designed a standard tripod type screw into the top, adjustable height, bar of the pack... so you can install anything from an arca rail to a yolk (yolk was the direction I went), etc.. so that your pack can be used as a very stable rest.. not just from the prone, but also from either the sitting or kneeing position (stand the pack up, and place the rifle in the yolk just like you would with sticks.. take a seat.. and then shoot...)...