In ancient computing times, which is when C was birthed, the size of integers at the hardware level and their representation was much more diverse than it is today. The register bit-width was almost arbitrary, not the tidy powers of 2 that everyone is accustomed to today.

The integer representation wasn't always two's complement in the early days of computing, so you couldn't even assume that. C++ only required integer representations to be two's complement as of C++20, since the last architectures that don't work this way had effectively been dead for decades.

In that context, an 'int' was supposed to be the native word size of an integer on a given architecture. A long time ago, 'int' was an abstraction over the dozen different bit-widths used in real hardware. In that context, it was an aid to portability.