The main reason why people leaving duties in progress is not that they only want to join a nearly finish duty. If they want that kind of duties they'll know before joining since the DF window will tell them that it's 2/3, 3/4, etc.

It's because they examined your gears and they think it's not worth playing with you. joining duties in progress will be ok if you join the ones that doing good but had someone disconnected. a lot of parties ended up getting wiped at some point so some member ditched the other members, if the Duty Finder send me to a party like that I'm gonna leave too and I don't think I've done anything wrong doing so.
Yes but this thread isn't about people leaving the dungeon halfway through, it's about people spamming withdraw in DF until they get a group in progress.