Why in the lands of Europe, captured by the Goths or Franks in the IV-V centuries, the Roman economic system changed dramatically?

The Goths and Franks of the 4th-6th centuries did not own urban civilization and did not want to adopt it from the vanquished.

Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.