OM-D E-M1 Mark III RAW images are not supported. why?

Just to add, Apple only ever updates its Camera RAW database for its most recent OS, that being at the moment Catalina.

This is the latest list, your camera is still not included.

Digital camera RAW formats supported by iOS 13, iPadOS 13 and macOS Catalina – Apple Support

It is entirely possible that Apple will hold off supporting your camera in Catalina and it will be added to the camera RAW database in the next OS, Big Sur which should arrive late September or October.

As you are posting in the High Sierra community I am going to presume that you are running High Sierra.

So don't hold your breath hoping your Olympus OM-D E-M1 Mark III raw files will ever be supported

in this OS.

You can download the free version of Adobe DNG converter, this will convert your RAW images to .dng files.

(Digital Negative), having done that then you will be able to review the files in Preview, Quicklook and Photos.

It is free, https://supportdownloads.adobe.com/detail.jsp?ftpID=6969

Because Olympus has not supplied it to Apple as yet.

FWIW, you can view the thumbnails in XnView MP.

https://www.xnview.com/en/xnviewmp/