Great! Let's have a year of no new developer features and everyone sits down and documents the existing code. And not just "Peter the intern no one wants" typing in "This sets the density." as a comment for a function parameter named "density".
So relying on reading code also relies on having the people who wrote the code available to explain the “why”. Or documentation.
It's also a lot more effort to dive into the code to understand what a method does (again, this typically includes background information related to things outside of the method itself) than to read an ideally well-formed sentence that precisely answers the main questions about the method's usage.