Interesting discussion!
I think it's true that, in a survival situation, good enough is good enough. If I were lost in the woods and starving, I could whack down a green chokecherry branch, string it with a shoelace, strap a couple found feathers to a reasonably straight, green shoot, and probably make something that would kill a deer at close range. It wouldn't last long, but if I lived in an area with lots of straight chokecherry branches, it might be more efficient just make a new bow every few days and never bother with finely crafting one out of seasoned wood. I read somewhere that the extremely long, d-cross section English longbow design at least partially came about because good bow wood was fairly rare, and with that design it's easy to mass-produce a whole bunch of bows from one tree trunk (compared to flat bows, sinew-backed bows, etc). In a primitive situation, I would think that the making of artistic, finely finished, high performing bows is something of a luxury. If I'm really depending on archery for my life, my design has to be both effective AND efficiently cheap (in materials, time, resources) to build.
Here's another aspect to this that's already been mentioned: I think it's entirely possible that sometimes, the people one sees in such pictures aren't as "primitive" as the pictures would indicate. How many people in developing countries will wear their animal skins and carry bows around while the tourists are in the village, then put all that stuff away and get back into t-shirts after they leave? Sometimes, when I see a picture of someone holding an obviously substandard, but very quaint-looking, piece of gear, I suspect that's what's going on. It's probably pretty common.
