Why does `free` in C not take the number of bytes to be freed?

Question

Just to be clear: I do know that malloc and free are implemented in the C library, which usually allocates chunks of memory from the OS and does it

Answer 1

C may not be as "abstract" as C++, but it's still intended to be an abstraction over assembly. To that end, the lowest-level details are taken out of the equation. This prevents you from having to furtle about with alignment and padding, for the most part, which would make all your C programs non-portable.

In short, this is the entire point of writing an abstraction.