I have this feeling that most current web novels (except some of slice-of-life genre) are lacking in fillers. Not boring info dumps or irrelevant villains that appear out of nowhere and disappear in 2-3 chapters, but something similar to usual side stories that you can find after novel is completed or anime-like fillers. Filler arcs can be good too. I'm not talking about fillers in the middle of the plot, but every time when huge arc ends it will be nice to see something casual. MC can try some hobby, go sightseeing, finally accomplish his wish from previous life (99% of MCs are transmigrators for nothing) etc. It doesn't even need to be all that original, they can do like usual - steal others' ideas. For example in xianxia world MC can open his own restaurant and try to reach the top of industry, some other forms of business management, he/she can use illusion techniques to reproduce some films or create VR game etc. It can also be something short like fan service anime episodes(shopping, beach, onsen) or naruto-type fillers that introduce some side stories and unknown characters that were only mentioned in world building. Idk why WN authors don't use such things. Reading some WNs is extremely tiring and kinda boring. No matter how good plot is, MC will still smash through everything using his 2-3 long overused skills. And after the end of some long intense arc author will insert a line 'after resting for 3 days MC went to the next 'dungeon', so here we are now'. That's also the reason why I'm somewhat sceptical about all the novels without any romance tags. Not that I'm looking for romance in the stories, but more often than not it will be a one track minded read with MC's sole pursuit of power, 0 character development, shallow side characters and repetitive spam of the same skills. Even worse, when WN authors think about fillers themselves it's always info dumps and annoying young masters. At times they can simply repeat something that already happened once again. I wonder why such things are rare in CN/KR fantasy? Any particular reason? Maybe they tried and failed? It's not like they are forced to write the same old things, the older WN industry is, the harder it is to find something that wasn't used before, so why not mix genres? (in moderation ofc). No matter what they write it can't get any worse than another dungeon clear or arrogant young master arc, so why not put some focus on character dev, side stories or whatever. Readers will be able to see something new and get some rest from all the fighting, authors can use this time to think of better ideas for future serious arcs etc
I'm not sure I would ask for full on filler arcs or side stories, but I do agree that having some downtime in between the action and/or drama is generally a good thing, both for the characters and the readers. But well I'm not sure I've encountered more webnovels that have too little of it compared to ones that have too much of it, depends a lot on what stories you read I suppose. As for why some author shy away from it, it could be a lot of things but I would believe at least a small part of it is that some authors that write power fantasy plots might find it boring to write other things.
https://www.novelupdates.com/series/banished-to-another-world/ has good interlude chapters. Idyllic without being boring. Narratively, it makes sense characters want a day off from the daily grind.