Because pre fab coops/runs are generally very poorly built. (THAT is why they are cheap.) Cheap materials that don't last, now where NEAR enough ventilation. If you are just looking for something to "get by" for a little while while you build something else. It would probably be fine. But...
@DobieLover How far should the roost boards themselves be from the wall for large hens? (I have turkeys so mine is about 26 inches from the wall so as not to mess up their tails. But I know that is way more than necessary for chickens since they are much smaller. )
Pictures of your setup would help. It the run is tiny, and you get a fair amount of rain, it will likely turn into a muddy mess in no time if you don't put something on the ground. But if it is large and/or you live in the desert like I do, mud likely won't be an issue.