Extended attributes are just that... attributes. They are not the main "data" of the file. Each and every unix file system has some attributes to begin with (permissions, timestamps etc.) and they vary from filesystem to filesystem (e.g., setuid bit, immutable bit etc.). User supplied attributes just extends this concept in a natural way. All the standard tools expect to be able to call open() on the file and start reading a stream of bytes assuming there's sufficient access. That's what a "file" is.
Moreover, it's not just unix as an isolated systems. Those expectations are baked into the structure of the entire Internet. When you receiving a "file" over any medium like email, web etc., you're expecting to receive the aforementioned stream of bytes. The attributes (or extended attributes) are not expected to accompany the file data as a general case.
Resource forks on the other hand just completely work against reasonable user expectations. The example given in the OP's post is one such instance. A font file that shows up as having zero bytes to every tool that works with files including tools that expect to transfer "files" over the internet. It's just broken by design.