Because that would take a long time if you deleted a large file in another partition or drive. You could also end up not having enough space to move the file to trash and if the trash directory is on an SSD, it would add a lot of unnecessary wear to it.
I would get a recertified enterprise drive from Server Part Deals. Drives in the 12-18TB range currently have the best price per TB. Be sure to get a SATA drive if it’s going in a desktop.