Layers of abstraction remain effective and valuable. Why reinvent state management, for example, with each application?

Runtime also matters; you can’t run assembly on the web.

Security mechanisms can also preclude assembly.

Etc.

FWIW, your question stopped short before the bottom turtle in the stack. Below assembly is machine code. So your question could rather be, why not emit machine code. Assembly is made for humans because we can understand it, but machine code is not really tractable for humans to engage with in a meaningful way.