Some people in this game are extremely lazy and want to play as little as possible. Back when we didn't have a penalty for withdrawing from a queue tanks would just spam withdraw over and over for hours trying to fish for a mostly finished in-progress run that they could just jump into for a quick clear, which made it terrible for everyone else and took longer than joining and running the content themselves would have.

I think now people worry that a group will kick someone so their friend can join in-progress, or some other manipulation of the system. I don't know if that would actually happen enough to be worth preventing and it's difficult for me to comprehend people encouraging and assisting in their friends lazy good-for-nothing behavior.

It would be cool if we could just have nice things, but somebody will usually find a way to ruin it for everyone else.