Why should I run my OS on an SSD if it means no RAID on a HP Microserver?

I have to install a HP Microserver Gen9 with Windows server OS.

I read everywhere these kinds of servers are set up like this:

• OS on the separate SSD.
• Data on the other 4 HDD in RAID 1.

The thing I don’t get and I cannot find an answer to:

This means the OS is not in the RAID, so if that SSD breaks there is no RAID backup.

Why not place the OS in the RAID, if speed is needed make the RAID in SSD’s? Or; what is the benefit of an extra-not-part-of-the-RAID SSD?