Because of how unraid implements parity, staggered spinup is not able to be implemented without major design changes which are unlikely to be made.
When you first power up your system, the controller is in charge of spinup, and can manage it. However... after unraid is in charge, the drive controller no longer has any say in when drives are spun up or spun down. If you implement spin down for drives that have not been accessed, which is one of the major selling points of unraid, then when unraid gets a disk access request it will spin up the drive asap.
This is all fine and dandy when your power supply is properly sized for the number of drives that need to be spun up at once, which is ALL of them.
Picture this. Your array is idling, only one disk is spun up and reading, and a read error occurs. The first thing unraid does is recreate the data that should be in the spot that errored out from parity. That means ALL the parity array drives are immediately asked to spin up. If your power supply is not able to handle that, you will get multiple read errors, and possibly multiple disabled drives.
So.. here is the TLDR;
You either need to size your power supply to be able to spin up all your drives at once without any stress or issues, OR disable spindown so the only startup surge is on initial power up and can be managed by your controllers. Since most people that use unraid want to spin down unused drives, the normal advice given is size your power supply appropriately.