What is the reason behind the belief that universities only provide basic knowledge and do not teach anything useful?

You are missing the point, the purpose of a degree is to certify that an individual has an aptitude to learn a given field of study. Nobody is expected to be an expert on their first day of the job after earning a degree. What is expected is for a new hire to learn how to be productive in a short period of time, and the level of productivity is expected to be far greater than a candidate without a degree. While the degree may not provide specific skills necessary for a given job role, they should be close enough for the person to learn them quickly.

-Godspeed