Nations as a concept didn't exist when those empires were founded. And it's really a trade off. Within the boundaries of the empire you had peace and prosperity. It was on the frontiers that people suffered.
Nations as in "a group of people united by common descent, history, culture, and language, inhabiting a particular state or territory" most certainly did exist.
> Within the boundaries of the empire you had peace and prosperity.
Then you don't know much (or anything) about the life of people conquered by caliphates.