I believe it's cause the entry is made by people who want people to know it exists, or people who plan on uploading it, but didn't finish their translation yet (does happen occasionally)
and to respond to the other issue, the main issue with tracking translation progress is that mangadex supports multiple languages, which means it'd need to differentiate between all of them, which is extra work to set up and design around (not to mention skipped chapters and whatnot, also hampered by weird chapter numbering and other issues), so it just never became a thing. Actual series progression is at least viable to track, and can tell you how many chapters will be there.
There's also other issues of chapters being retracted, and once again, other languages (tho in this case, no chapters in any language have been posted yet, or they've since been deleted, I wouldn't know).
And personally, I believe there is more benefit to being able to create titles without needing to post chapters since it'll at least mark it for sake of the records (on the author's page for example) and help people know of it, rather than make some uploader do it, who knows when, if they even know it exists