25 years ago people would have said the same thing about OS kernels, and yet here we are. Some of the underlying problems I see are that the game industry is still young and hasn't really standardized everything yet. Because of that, there is still ruthless competition between the hardware and GFX card vendors. Vulkan is a step in the right direction, but it will obviously take time for it to become adopted.

A lot of it is also driven by the popularity of consoles and mobile devices which are still largely locked-down walled gardens. They intentionally make it difficult to share code between platforms too. But, soon enough game developers will get sick of writing the same code over and over again, just as OS developers did in the 90s. Once this happens you'll start to see a lot more ad-hoc code sharing which will pave the way for more engines being made free software.