It's also industry standard to do so. I don't think I've ever seen a team outsource something like pathfinding. Maybe in the Unity/Unreal space, which I'm very unfamiliar with. Dependencies are generally speaking not viewed as a good thing. They become vanishingly rare outside of certain things like physics engines, sound engines, vegetation, etc. And usually higher quality proprietary ones are chosen over OSS (though this is changing with physics engines in particular, mostly thanks to Bullet)

NIH is a cultural pillar. Even scripting layers are relatively split on if they're in-house or not. It's not uncommon to find both an in-house fork of Lua + a few other completely custom scripting engines all serving their own purpose.

Everyone will have their different take on what Swift is, and what a proper alternative looks like. Swift started life as a C-family successor, so I'll be looking at languages with similar aims (fine-grain control, scaling to large projects.)

The first two I'd mention are D and Nim. I only wrote ~10k LoC with these languages (so they weren't really for me) but they strike me as similar to Swift. They both optionally support an automatic memory management strategy like GC, (whereas Swift as ARC) and there is great effort put into metaprogramming facilities. Both D and Nim compile much faster than Swift, and offer better error messages than the Swift compiler in the presence of complex generic expressions. In the context of the parent post (games) Nim is especially well equipped with a package ecosystem. D seems a touch less lively, but has a following.

For myself, I prefer the Odin programming language. Full disclosure, I've been donating to Odin for about a year now after happily using it for more than 5. After writing "Orthodox C++" for a while, I stumbled on Odin and feel as if it was made specifically for me. The compiler is fast, and the language has been something like a tutor or mentor for me, as it applies friction in places where I usually waste time. It would take some hand-holding as a first-language though, as some error messages relating to overloading/generics would seem obtuse to a beginner.

EDIT: I've recently been exposed to the Raku language, and it strikes me as sort of a ... dynamic version of Swift? It's jammed full of functionality, with an emphasis on being able to design syntax a-la DSL.

Also I'd add that Swift is sort of part of this new litter of "no-paradigm languages" like Kotlin and C#. I don't think Kotlin can actually dip as low-level as C# and Swift can, though. At least I think only C# and Swift have some kind of safety-wrapped user-level pointer "thing."