This single thing shows how out of control BA operations are when it comes to maintaining SOP's. The answer is either:

• The menus were not loaded with the catering
• The crew could not find them
• The crew could find them but they were at the back door and they didn't find time / couldn't be bothered to fetch them (or both)
• The loading of each choice was so limited they did not want to create an expectation that they "should" have had something when all that is left is the Vegetarian shoe sole
• The crew just couldn't be bothered

As something that is so visible to passengers (and therefore everyone), it is something that BA could insist is done without exception. That staff take matters into their own hands shows how little they care about doing the job and the consequences of being caught. As you say, it is not every time. Some crew really excel. Recently a return to IST had no menus yet a return to GLA had them both ways.