Because you also don't pay tax on the ticket, hence my calculations which bump it up from 1.67 to roughly 1.865.
You can try it yourself on their site and make your own calculations, they may very based on the route.
TBH even that's conservative, as I fly WN over 20 times annually, exclusively using points. And very often get a value of 2 cents pp.
For example last night flew LAS -LAX for 2,055 points, whIle the cash price was 49$ , after 9/11 tax it's still 43.40. Which means a value of 2.1