That would require all of those disks to be connected at once which is a logistical nightmare. It would be hard with modern drives already but also consider that we’re talking IDE drives here; it’s hard enough to connect one of them to a modern system, let alone 12 simultaneously.
With an Index, you also gain the ability to lose and restore partial data. With a RAID array it’s all or nothing; requiring wasting a bunch of space for being able to restore everything at once. Using an index, you can simply check which data was lost and prepare another copy of that data on a spare drive.