Most hardware architectures (and software ones based on them) the compiler controls the calling convention: how the stack is managed, what gets pushed there vs passed in registers, and so forth. The architecture may or may not have helpers like specific "return from subroutine" instructions that help manage the stack... but a lot of things are under the compiler's control.
WASM is not like that. It doesn't support jumps or that kind of manipulation. It does not expose enough control over the stack and calling conventions.
In theory could the compiler emit a virtual machine that simulates another machine where it _can_ control these parameters? Sure. It can rewrite it all into a huge loop-and-switch statement. But that is not going to result in anything close to efficient native code. By expressing tail calls directly in WASM the JIT can generate much more efficient code that takes advantage of the platform's native calling convention and tail calls can be _actual_ tail calls on the hardware.
WASM is not like that. It doesn't support jumps or that kind of manipulation. It does not expose enough control over the stack and calling conventions.
In theory could the compiler emit a virtual machine that simulates another machine where it _can_ control these parameters? Sure. It can rewrite it all into a huge loop-and-switch statement. But that is not going to result in anything close to efficient native code. By expressing tail calls directly in WASM the JIT can generate much more efficient code that takes advantage of the platform's native calling convention and tail calls can be _actual_ tail calls on the hardware.