A historical parallel could be made to the fall of the holy roman empire and the medieval dark ages.

Essentially, centuries of endless conflict and bloodshed with no real victor has a reversing effect on human progression and achievement.

This isn't unrealistic either.

In order to have advanced technology you need to be able to distribute and specialize labor. That requires an economy where goods can be dispersed allowing individuals to focus on matters other than personal survival. This also requires a government/laws to protect markets and trade so that goods and services can be exchanged.

You cant have an Iphone factory if everyone is just taking the iphones.

After generations of such conflict knowledge becomes lost causing reversion of human progression. Then the final reversion is the loss of data in the form of actual databases/infrastructure to books and scrolls.

For instance: if somehow every electronic data storage device was simultaneously destroyed. The loss to modern human progress would be incalculable.

So in your world just have cultures that constantly clash violently and without any real periods of peace.

A historical parallel could be made to the fall of the Roman Empire and the medieval Dark Ages.

Essentially, centuries of endless conflict and bloodshed with no real victor has a reversing effect on human progression and achievement.

This isn't unrealistic either.

In order to have an advanced technology, you need to be able to distribute and specialize labor. That requires an economy where goods can be dispersed allowing individuals to focus on matters other than personal survival. This also requires a government/laws to protect markets and trade so that goods and services can be exchanged.

You can't have an iPhone factory if everyone is just taking the phones.

After generations of such conflict, knowledge becomes lost causing reversion of human progression. Then the final reversion is the loss of data in the form of actual databases/infrastructure to books and scrolls.

For instance: if somehow every electronic data storage device was simultaneously destroyed. The loss to modern human progress would be incalculable.

So in your world just have cultures that constantly clash violently and without any real periods of peace.