It's a cultural, psychological, linguistic mixup more than anything else. People do read code all the time, they just hesitate to respond to a question like "what code have you read recently". It's hard to answer that question in english without implying that you have completely read a program (rare) that was completely written (in other words, "finished", which is even more rare).
If you asked a different question, like "explain how you read code in the course of a typical project or experiment" you will get a ton of examples. They might describe how they look to understand the basic data structures, and then imagine some sample data flowing through the algorithm to understand the purpose, and then examine the details, edge cases, and interactions to see why some non-obvious choices were made. Then they might describe how they use this to find what parts of the code should be generalized, specialized, or extended to fit new functionality.
It might be interesting to incorporate code reading into an interview to see the strategies that people use. It would be quite difficult to make it a fair question, though, because patterns vary widely and it often takes more than an hour or so to adapt.
I think it is understood that we all read code during a typical project. I think what the OP is referring to is code reading for the purpose of improving your skill in general, to get exposed to code that you wouldn't normally see by just working as usual.
If you asked a different question, like "explain how you read code in the course of a typical project or experiment" you will get a ton of examples. They might describe how they look to understand the basic data structures, and then imagine some sample data flowing through the algorithm to understand the purpose, and then examine the details, edge cases, and interactions to see why some non-obvious choices were made. Then they might describe how they use this to find what parts of the code should be generalized, specialized, or extended to fit new functionality.
It might be interesting to incorporate code reading into an interview to see the strategies that people use. It would be quite difficult to make it a fair question, though, because patterns vary widely and it often takes more than an hour or so to adapt.