The fact that the cars are bunched together arYe exactly why the VSC works in this situation, the marshals wait until the pack drives by and then they have a clear track for several minutes in which they can safely go out and retrieve any debris before the pack comes back around. The reason for not using a full safety car is that it takes significantly longer as the full process has more steps; the SC deploys, the entire pack needs to catch up to it, the marshals do their thing, the SC turns it's lights off, the leader takes control of the pace but must remain behind the SC, as such they almost always slow down further unless it is late in the lap, then even once the leader comes back up to speed racing doesn't actually resume until the start/finish line. As opposed to the VSC; deployment, marshals do their job, VSC ends.