NEF and ARW are RAW file formats. They are not images in a way that they fully define an image. They are rather sensor plus meta data that can be interpreted into a well defined image. If you load them into different RAW converters, they will look differently.
Photo agencies sell developed and processed images, not RAW files. Plus most photographers would not give the RAW file away.
That would be like if Microsoft gave you the source code to Word instead of a ready-to-use application. Yes, that would be nice if you were a programmer, but for most people this would just be more work.
As RAW files need to be processed into fully defined images, browsers cannot display them. Additionally they are usually too big for sensible web display. Thus, wix and other companies do not support them.
Furthermore no image agency that I am aware of, sells them directly, unless for training purposes. In that case they usually convert them into a DNG as a meta format for better compatibility.