Just hit auto resolve. This game isn't DnD, where if you "hit" you do at least 1damage unless the monster has damage resistance. If the defense roll is higher than the attack roll damage=0 and "miss" pops up. It is a carry over from GalCivII I think. I know I would make some ships with only 1-2 attack and max defense and if they got in battle it would take 100s of turns for the battle to finish. That game didn't have tactical battles though, and I would just fast foward or auto resolve.
Does the system work in a game with tactical battles , magic and skill use? Maybe not. Over time the units with the higher stats should win more often than not. But I am not sure if that is fun.
I found it helps to look at attack and defense as a range rather than numbers IMO.
Instead of thinking 30attack/50defense think 1-30attack/1-50defense. Or average them 15attack/25defense. In a mirror match over half of the attacks would do zero damage. Just understand you can hit really low/high rolls too.
And yes if you took two units with 50def each and only gave them clubs, and told them to smack each other till one fell over - it would on average take 100s of turns even with only 5health.