WHHHYYYY?!?!

Likely to get more money or safe time?
Many authors of these webtoons charge per page, so leaving them short gets them more money. Or they are the type that safe themselves from more work by shortening a chapter so they can work less and still say 'I put out a chapter, don't complain'.
OR it might be both. I dunno. Anything's possible (inaworldsoseriouslystrange).

Short chapters are getting more 'popular' with even big-name manga artists (obviously not the fans tho), it's usually to cut the workload and still meet the deadline for a new chapter.