Investing in Swift would to an extent canibalize Go, Dart/Flutter, TypeScript, C#. You know, the technologies which have creators that actually care and invest billions in cross-platform development.
Not to mention fragmentation.
Apple doesn't even care about Linux running in their own hardware. Their contribution towards that goal ranges from hostile to scraps, depending on who you ask.
In fact, they don't even have to do that much for Swift. They don't have to develop neither the language nor the compiler.
They need to do similar to what The Browser Company did, exposing and allowing usage of their APIs from Swift, and maintain it as their API evolves.
If The Browser Company could afford it, while developing the browser, Google and Microsoft can afford it as well.
Commoditizing your own product is never in a company's best interest so don't expect it to happen willingly.
If The Browser Company could afford it, 3 trillion USD market cap Apple can afford it as well.