Porting a game to Linux and Vulkan does not require rewriting it from scratch.
It does because 99% of the game studios don't care about Linux, nor are that much into Vulkan, unless they are targeting Android or Switch.
The graphics backend is really not such a big cost for most games, especially with existing cross-platform engines. Also the grahics APIs are really not that different as they all model the same GPUs. DirectX vs. OpenGL had some annoying things but most of them are only a problem when you have to translate and can't modify the calling code at all. DirectX 12 vs. Vulkan is even less of a difference.
How many Android and Switch games get published on GNU/Linux? Yep, it boils down to it.
Let's cool it with the hyperbole, okay?
There are many reasons Android and Switch games don't get ported to Linux; having to rewrite the game from scratch is not one of them.
Which proves the point that even when games happen to be natively written against a Linux userland, with native support for Vulkan, it isn't economically viable to sell them on GNU/Linux.