Because an update to the compiler could come out next week that completely alters the language. If you're developing a product in this language, you'll need to put significant effort into keeping it up to date, and any dependencies you've downloaded in source form may not work on the most recent compiler.
In one such change, all *Allocator parameters were turned into Allocator parameters (not the missing *). That meant rewriting tons of function bodies and signatures, because passing specific allocators around is one of Zig's strengths. The compiled binaries came out just fine, but every major Zig component needed refactoring from one compiler version to the next.