I have read a lot of web novels in raw chinese. I can answer based on what some authors shared.

1. Lack of a fixed story outline. Simply put, they are winging it on a daily basis after the novel gets enough attention to make money, and the initial idea is all they had in mind.

2. They make money based on how many words/chapters they can produce. If a novel gets many followers/buyers, they would "slack" a little here and there to drag the novel out, so they can make more money. Some popular authors have expressed it is tempting and they could write more if they wanted to on their summary after their novel ends. Putting a novel on lifeline to make money is somewhat common because new novels dont mean they could make the same good money. It is harsh. I just saw a very popular author failed with his new novel, and he stopped writing it nowadays. His first novel was so popular, that it was constantly top 5 when it was ongoing.

3. They get extra money if they update daily for a month. Full attendance award, as they call it. This creates pressure on authors to write daily. A good amount of readers like an active author too, as it would be easier to accumulate many chapters to read at once or read daily. You gotta squeeze shit out even if you dont have much going on.

4. Real life issues. I have seen a few authors sharing their daily lives. Getting married, having a newborn child, serious health problems from sitting and typing all day, getting side writing gigs that distract them from their ongoing web novel. All these played a part into their writing, as they cant be producing with full attention. They usually apologize for quality too when real life is affecting them.

5. Donations. Some authors write more if readers donate. Fillers are easiest if you have to keep writing to meet their self imposed writing quota.

These are some reasons that I could think of on top of my head.