Why isn't anyone testing candidates' ability on reading and working with existing code? I do that all the time with interviewees. Why I do that? The obvious be

Why isn't anyone testing candidates' ability on reading and working with existing code? I do that all the time with interviewees. Why I do that? The obvious being the potential hire is required to work within a large code base (often code not written by yourself) and having the ability to read and understand quickly what a piece of code does is a great skill that not many great coders have.