Do people actually think that? They must have never tried good Vietnamese cuisine. You find plenty of tasty, healthy dishes in Thai food as well. Not to mention a simple salad... Where does this myth come from?
A salad, to a lot of people, might not taste very good. But adding a pinch of olive oil and some lemon juice might make a world of difference.
I guess people see those "organic" material in the store with minimal branding and then compare it to a McMeal that looks amazing.
Whenever I cook, I try to make the dish taste great, smell great and look good. All of those are important in a meal. Obviously I fail a lot, but those are my targets in creating healthy-delicious meals that don't cost a fortune.
Someone I know recently tried to make a "healthy" meal by combining lentils with oats and just cooking them in water and throwing a small pinch of cumin in it. Now that might be a "healthy" meal but it tastes like sawdust...
Same reason bad-tasting placebos are more effective than sweet placebos.