@Kampfarsch
The web-novel is split into five "Parts" which are basically just arcs.
The LN resets its numbering at the end of each Part, so for example they don't call the seventeenth volume "Volume 17" they call it "Part 4 Volume 5". This convention is carried over to the manga. Look at the covers of the final volume of this manga and then the first covers of the sequel and the sequel's sequel
, in particular look at the numbering. So it makes sense to have separate entries for each Part. Then to add to this they all have separate Amazon listing groupings (although Bookwalker is just one) and separate raws links, which makes separating them sensible as it shows it's Japanese distinction.
But also there is an issue if they were just one entry and that is how do we order the volumes and chapters? What is "chapter 34" and what is "volume 8"? The immediate answer would be that chapter 34 is the first chapter of Part 2 (in spite of it being labelled chapter 1) and that volume 8 is the first volume of Part 2, but this isn't so clear cut because chronologically chapter 1 of Part 3 came out first (before Part 1 even finished!) and volume 1 of Part 3 came out before volume 1 of Part 2. Then to make matters even worse Part 1/2 and Part 3 are drawn by different people and both Part 2 and 3 are still being published. So grouping them all as one makes no sense.
It's feasible to group Part 1 and 2 together and have Part 3 separate but then that looks weird, ignores different Amazon groups, ignores different raw links, ignores different AniList links, etc. So I'm wholeheartedly against merging Part 1 and 2, and think that the way MangaDex is doing things is the only sensible way of doing it.
Now you may be asking "Wait... Why is Part 3 being published at the same time as Part 2? Don't you need to read Part 2 before Part 3?" There are two reasons firstly the LNs have long since completed Part 2 so LN readers can freely read Part 3, and the second is that this series is very long, this manga page we're on only adapts Part 1 which is just
% of the whole story but it took Japan 3 years to finish it! Two other manga that have done this are Re:Zero and Mahouka.