Wasm is too high level to implement your own tail calls. The WASM virtual machine handles the call stack & function calling convention, instead of being something that the code itself is responsible for. This means that the compiler can't implement tail calls; WASM doesn't allow a jump instruction in one function to jump into a different function.

There are a bunch of reasons why WASM was designed to be higher level than native assembly. One is that they want it to be more compact to save bandwidth when downloading via the internet. Another is that they want to make it faster to compile down to native code; one way they do this is that the control flow is more structured, for example it forbids irreducible control flow graphs. There is also the matter of safety. WASM defines several validation constraints that are checked before the bytecode is run e.g.: the target of a jump instruction must be listed in labels list. If WASM were too low level, it wouldn't be possible to do the safety checks they want.