Specially because it's not a drop-in replacement for C++. As Zig is for C.
So when Zig hits 1.0 these companies will probably consider Zig much more than they do today. Understandably.
Secondly, let us know when Amazon rewrites Firecracker in Zig, Android replaces Rust with Zig, or Mark Russinovich goes to some Zig conference explaining why Azure is dropping Rust for Zig,
"Mark Russinovich, Microsoft Azure CTO tells Rust Nation UK 2025 why Azure is moving to Rust from C++"