two factors to consider

Professions have great influences in building one's charactor.

A good engineer can have a very good career. Simply being good as a mathematician or physicist is usually not enough. You have to be excellent, even execeptional. Logically, only a small percentage of the mathematicans and physists are exceptional. Most of the left majority feel lost, fall into depression, and isolate themselves from other people. To be worse, the job market is not favorable to them, in the sense that there is only a small number of positions are available. So most of them work for low-pay jobs, if they could get one. As a psychological defense, or you can call it a vanity, they put on a face. They definitely have other choices, but most of them have never learned, or even realized, that someday they might have to deal with this situation. The education systems in China was definitely not helpful. The top students were mostly spoiled by the system and the parents. They had thought everything would be fine sinec they are super-smart and work hard. They didn't know real life doesn't always work that way. They were not taught about this, how to handle this, how to work with others, etc.

There is also sample bias. To make the argument simple, let's assume there are equal number of students between PKU and THU each year. Among THU student, let's assume 75% would end in engineering. Among PKU students, those majoring in math, physics, etc. is a much smaller percentage. It can easily leave a disproportional impression.

As a last note, I found those who have good careers seldom talk about their colleges. I usually avoid those people who make sure to let you know they are from a prestigious university, whether it is PKU, THU, or they went to college in Boston :)