Incentives to have grad students finish

In the home country, the time to finish your PhD is four years. From the start it is clear that you will need to finish within that time, because then the money has run out. I have to add that you can only start a PhD program when you already have a master’s degree, so there’s a lot less course work than in the US. Also, truth be told, most people take longer, but you have to ask your PI to support you longer, or write your thesis on welfare money. Also, departments usually get money (I don’t know how much) when grad students graduate, so that’s an incentive to get grad students to finish too. 

However, in the US it seems that there are very little incentives for grad students (or their PIs) to finish their thesis and defend. For PIs, keeping a grad student around a bit longer seems only beneficial: it’s a trained person who costs relatively little. Or, what I recently see around me quite a bit, is that PIs just seem too busy to read a grad student’s manuscripts and that keeps them around a lot longer than necessary in my opinion. And from the grad student’s point of view I guess sometimes it’s nice to stick around a bit longer to wait for job opportunities (or the lack thereof).

So what am I missing? What are the incentives to finish your PhD in the US, either for the grad student, the PI or the department?