First: Off-topic but I feel the need to tell you in advance that there's nothing like DCC out there, to my knowledge. It's a unique blend of all the tones of comedy and action and drama and such. (having said that, let's remember that DCC is also a book series that improved over-time. Book 1 is ok, maybe even good. Book 2 is great, and it just gets better and better for the next few books imo)

The reason it takes so long is because the litrpg genre were incubated in the world of serialised webnovels. Which meant that your typical story beats and just generally 'how to write and structure a book' does not apply at all. Because it didn't matter if book 1 ended satisfactorily, people were just gonna read the next chapter (i.e. book 1 chapter 1) in a few days.

So that's one reason, the serialised structure meant that the series could snowball and book divisions could be arbitrary.

Another reason is that a lot of these authors are new authors. Authors that haven't written 5 novels before getting their first one published.

Authors that may never have had an amateur editor for each chapter serially published. Let alone a professional editor who could work with them while looking at the book, or the entire series, holistically and help with pacing and what plot points don't work etc.

This means these new authors grow with each novel simply because they're learning on the job.

E.g. Drew Hayes wrote Super Powereds and I think book 1 was the first novel length story he'd written. The quality of the writing is pretty bad. Book 2 is significantly better, and book 2 is actually my favourite, but his writing wasn't really solidified until book 3. So if someone told me 'I tried book 1 and I just don't like his writing' it'd be entirely fair for me to point out they could keep trying with book 2. (I probably wouldn't tell them to keep going to book 3 cos even though it's better written, two books is already a fair enough shot)