The main reason is that it simplifies production -- only one software image is needed regardless of the SSD shipped with the system, rather than needing to have different drivers for Samsung, WD, Crucial, etc. SSDs.  
Having Raid on a single SSD is crazy. In my testing of very fast SSDs like the Kingston KC3000 can lessen 25% 0ff its performance. The only benefit to Dell is that users will not be able to change the SSD without first going into the bios and changing the settings from Raid to ACHI. As Raid hurts performance over stand-a-lone single SSD drives, why does Dell Tech want to still persist in its adoption especially when the latest G15 has only 1 SSD slot?
Inspiron G15 5521 SE, one drive, Raid 0?