So I've worked as a classroom teacher, and like probably everyone whose ever had that job I learned in my first week in an actual classroom that students will almost never read the actual instructions of an assignment, say a worksheet, no matter how clear and strategically bolded they are, if based on the layout of the assignment or some other extrinsic factor they think they already know what they are supposed to do with it. An RPG is different in that they seem to disproportionately appeal to the sort of people who do actually read the rules, and usually the knowledge of commonly overlooked things actually written in those rules gradually filters out to the rest of the community. But otherwise I think the dynamics are basically the same.
Very few people read a book like the PHB word for word. Rules that run counter to common assumptions people make based on the presentation will routinely be overlooked or misunderstood. In a system where you have to choose a race, a class, and background and then there is a section with extensive options for each, and the prior two choices don't have a presumption of being able to create your own custom option, people are naturally going to think they're supposed to pick one of the set options. People are particularly likely to skip sections that seem like boring front matter that are in the way of them reading about all the neat backgrounds.
That being said, since the consequences of people not reading the instructions thoroughly in this instance are just that they have a background that's maybe not as much to their exact tastes as the one they would have invented, I think the accessibility benefits of listing out pre-made backgrounds far outweigh the consequence of a large segment of players thinking they are mandatory.