Because WASM sandbox only works, to the extent hackers have not bothered attacking existing implementations to the same level as they did to Java applets, which is anyway one implementation among many since 1958 UNCOL idea.

Additionally, it is a kind of worthless sandbox, given that the way it is designed it doesn't protect against memory corruption, so it is still possible to devise attacks, that will trigger execution flows leading to internal memory corruption, possibly changing the behaviour of an WASM module.

> Nevertheless, other classes of bugs are not obviated by the semantics of WebAssembly. Although attackers cannot perform direct code injection attacks, it is possible to hijack the control flow of a module using code reuse attacks against indirect calls. However, conventional return-oriented programming (ROP) attacks using short sequences of instructions (“gadgets”) are not possible in WebAssembly, because control-flow integrity ensures that call targets are valid functions declared at load time. Likewise, race conditions, such as time of check to time of use (TOCTOU) vulnerabilities, are possible in WebAssembly, since no execution or scheduling guarantees are provided beyond in-order execution and post-MVP atomic memory primitives :unicorn:. Similarly, side channel attacks can occur, such as timing attacks against modules. In the future, additional protections may be provided by runtimes or the toolchain, such as code diversification or memory randomization (similar to address space layout randomization (ASLR)), or bounded pointers (“fat” pointers).

--> https://webassembly.org/docs/security/

Finally, WASM is only as secure as its implementations, whatever the bytecode promises only matters if the runtimes aren't exploitable themselves.