This is very common for archive files. It lets you easily append a file to the end of the archive (overwriting the directory) followed by the updated directory. If it were at the start, you’d have to rewrite the entire contents of the archive to grow the directory.
Isn't the point of compression is to eliminate such wastes of space?
Maybe. But Unix types have always been all-in on tar. They love tar. They continue to use some form of tar to this day.
This was actually a 'cultural issue' back in the day, because Sun Java JAR files were just ZIPs. For good reason. But that wasn't the blessed holy unix archive format.
Vs. the folks creating and testing filesystems would have to do a whole lot of work to add that feature. Followed by the folks doing higher-level software, most of which which need to handle files being prepended to.
In short - interesting idea, not worth the effort.