I think there are several reasons:
First, the university is a place where people from all corners of the country get together and become classmates. There are great differences in diet, life and language, while the friends made in high school are generally in the same place, which is similar in many aspects, and there will be more topics. Therefore, it will make it difficult for our college friends to have in-depth communication, that is, there is no intimacy with our high school friends.
Secondly, the friends we made in college usually spend a short time together and have a shallow understanding of each other, so it is difficult for them to open their hearts to talk, while friends in high school, because they have spent a long time together and know each other better, will be more likely to confide their true thoughts to each other.
Thirdly, with the growth of age, we are less and less willing to spend a lot of time on making friends, which indirectly leads to the low quality of making friends in universities.
In this regard, I put forward such a suggestion: you can spend more time looking for like-minded people, deepen your understanding of talents, and then become real friends. Don't be afraid of not being rewarded. Making friends is not a utilitarian thing. Try to express your heart, communicate with others and show yourself more, so it is easier to make high-quality friends.
I hope my answer can help you!