I think the answer to that is they don't just want to simply port Witcher 3 to Mac/Linux, but the entire engine. By doing so they wouldn't have to:
1) Outsource every game they release.
2) Depend on the porting company for updates and optimizations.
3) Waste money for each game ported.
It would open up day 1 Mac/Linux releases and many other advantages, you probably get the idea already.
Porting this engine for Mac/Linux might be harder than one thinks, if they go with OpenGL it's well and good, supported on all OS's. With Vulkan however it's Windows/Linux only, so CDPR would have to either use Metal or good old OpenGL and that's a lot of work right there.
NOTE: Despite eON being similar to Wine, it has one major advantage: can execute DX11 games (such as Dirt Showdown) whereas DX11 on Wine is still in very early stages.
I didn't know eON could execute dx11... now I really cannot understand why they still wait to port the witcher 3.
Also, there is no point asking why they didn't use Unreal and etc. They are interested in investing in their own engine which they can develop any way they want, have expertise in it, and even license it to others (they specified such plans explicitly). So that's exactly what they are doing. As @Ganni1987 said, outsourcing such work to others is a bad idea. Not only you don't accumulate expertise - you have to go through the extra overhead and probably end up paying more money anyway.
Also, I expect because of all this Metal mess, and Apple refusing to support Vulkan in addition to stagnating OpenGL there, OS X will soon become very unattractive platform for developers.
Well OSX has always supported opengl, but it has always used an older version. I think they will port Vulkan but only after some years, so to standardize ios and osx they decide to port Metal to OSX too. I don't think they will end open gl support, but since they wrote metal for ios they decide to port to osx because it was ready.
I don't think Indie game developer will change their release because of Graphics API, most indie developer already use premade graphics engine and most program don't need Vulkan... Also since iOS have a good market and you need a Mac to develop for iOS I think OSX will continue to attact some developers.
I understand that they want to create a engine to license it, but they can not compete now.
Now there are 2 good game engine that are free to use, and can export for lots of platform.... and that engine can be used on Windows, Mac and Unreal can also be used on Linux (Linux version of Unity3d is in beta now).
There is also some good paid engine (like cryengine) that can export for many platforms.
And they want to compete in this market with their engine that only support windows development, and windows+ console export (and that you cannot use for mobile development)... As a developer I do not understand why one should choose their engine (aside for marketing reason... like: "this game use the same engine of The Witcher 3.... so buy it... LOL".
They are trying, but they are already late. As soon as they have a good engine it will be already old (just look how many features got the last update of Unity3d... and look how many feature they still want to update... just watch the difference between version 5.2 and 5.3, and between 4 and 5, and between 3 and 5... In a few years they have made many steps forward. Same can be said for Unreal Engine.
I don't think CD Projekt are able to make a better engine.