The sibling comment is one reason, but I'd like to add another. ext2 was introduced into Linux in Jan 1993, which is less than 18 months after Linux was first released. ext3 was made by copying the code from ext2 and renaming the identifiers s/ext2/ext3/g, and ext4 the same way from ext3.

So these filesystems are old. Even if it was a good idea to structure caching and filesystem block access separately (which as the sibling comment says, it isn't), the Linux developers were just getting started back then and it was much more important to get something working than to think about good abstractions.