The cars become cheaper to VW the longer they drag their feet.
The typical VW TDI owner likely drives more than 12,500 miles a year, which is why many of us bought TDIs... they make fantastic long distance driving vehicles and we can really pile on the miles, especially since diesel drivetrains tend to be more durable than gasoline drivetrains and thus better to withstand accumulating high mileage usage.
The vehicle buyback value is partly dictated by the miles on the vehicle at the time of turn in (not what you really had on the odometer in Sept 2015). So, if they keep stalling, a significant percentage of the vehicles end up being calculated at a lower buy back price. If just 5% of buybacks (of all 480,000 cars) are held up long enough to drop the buyback value of the car by $500, VW saves itself $12million. That isn't huge in terms of the overall settlement, but VW is looking to pinch pennies everywhere in this process... and that includes stalling you so they pay you less. Why do you think they understaffed the buyback positions as well to start with?
And you still don't think that there is an incentive for VW to stall?
Does the settlement spell out what happens to VW is they don't meet the 85% goal by Sept 2018? VW is betting that they would be granted an extension at that point. Because, why not? The courts almost always grant extensions as long as someone is perceived to be working towards a goal and doesn't give the appearance of running a sham operation.
All of these stories about the horrors of the process and the lengthy timelines also serve to discourage other owners from entering into the buyback process now. So some people, who would otherwise be participating right now, have decided to sit back and wait (and keep accumulating miles, perhaps at a rate that drops the value of their cars). These are the lazy people that have now lost their motivation because of the hassle of the process. You can bet VW was counting on that too.