That's because of sunk-cost rather than design.

Thanks to LLVM and GCC you can happily write embedded code in a higher level language, but the vendors don't bother supporting it because a lot of embedded coding isn't really what we would call software (no tests etc.)

Toolchains are one side, but garbage collection and big standard libraries are also a big reason. Anything with under a MB of RAM has a choice of several modern languages, but it is still basically just C, C++, Rust, Lua or MicroPython.

D works fine on microcontrollers.

I don't think anyone was going to write their fridge's code in Haskell anyway.

The higher level languages are kind of the problem though. I need things like the ability to know the layout of my structs.

Rust, D, Whatever let you control the layout of your struct.

Even C#, actually!