That could be for a wide variety of reasons. You might be looking at sellers using wholesale models on these sites or even wholesalers selling to the public direct. Many dropshippers run into these problems because it's very hard to compete on price when trying to compete with cashed up sellers buying in bulk.
What you do need to do is to contact the supplier and confirm the dropshipped price because many dropshippers also sell to the public and the prices you see on there website often reflect their retail price, not their dropship price.
So always first confirm the price with the supplier and then you will know if you are able to compete. If not, the only option is to keep searching for markets where you can compete.