Yeah, this is a significant barrier to flight being a viable tactic. Additionally, it can be very difficult to tell when you’re in a situation where you need to run away. In my experience it’s usually only in retrospect that you realize “we should probably have retreated two rounds ago,” and by that time you’re probably going to get TPKd anyway if you try to retreat. So, might as well keep fighting and hope you get enough lucky rolls and the DM gets enough unlucky ones that you somehow pull through.