This cliche is based on many professors' lived experience that students often (and repeatedly) ask questions that have already been answered by the course syllabus. This refrain (turned meme) cropped up when a professor wore a shirt bearing the phrase. A quick google image search turns up redundant memes displaying frustration over unread syllabi. Often a syllabus is best used to deliver a schedule of courses and due dates allowing the previous 14 pages of policies to fall by the wayside.
As is often pointed out when these grievances air, the syllabus is a Byzantine document the eschews any sense of design, audience, or engagement for the sake of delivering the bureaucratic minutia of a class and university. Syllabi are boring to write, boring to read, and recede into the periphery of a course once content exploration begins in earnest.
It is understandable how we got to a place where a syllabus is simultaneously essential and useless: An increase in these of litigious language and policy talk has helped protect and preserve faculty power in the classroom in an age where tenure is fading and students have begun to be seen as consumers rather than learners. A well-ordered syllabus provides a fall-back position for when things go wrong in the classroom. However, if the raison d'être of the syllabus is to make all members of a classroom informed about the boundaries and expectations of the class then it is a genre that seems to be missing its core audience. So low are the expectations of reading the syllabus that many students have become accustomed to a first day of class where they get told about the syllabus and then leave early without ever engaging in course content (as an aside, searching twitter for #syllabusday emphasizes this point quite well).