They will have read their way through their discipline. It isn't like this. Being a student is a job like any other. Ultimately, it's not about consumption but production. The purpose of reading is to gather materials in order to write.
Good students know what to read, and make their reading serve the specific ends of writing.
Of course, it's desirable to have time available for more leisured reading, for 'reading around' your subject. The joy and inspiration would be lacking in studies which were entirely instrumental. But given a crowded work schedule, especially if you're a part-time student with professional or domestic responsibilities, you often have to be very ruthless. Secondly, you must examine carefully the proportion of your time devoted to writing or its active preparation. Odds are, it isn't enough.