As to why Microsoft is charging 99 cents for the pack it is likely to do with licensing. Last I checked, Microsoft is not operating a deep-space telescope, and such images in 4K are rarely free.

It turns out that Microsoft's design team has rendered an updated 4K version of the default Windows XP wallpaper —you might know it by its name, " Bliss." ...

Microsoft Design - Wallpapers https://wallpapers.microsoft.design/ After accessing the public page, scroll down the screen. Scroll to the bottom of the screen and you will find 'Windows XP Wallpaper'.