Why does getpid() return pid_t instead of int?

I think it's the opposite: making the program portable across platforms, regardless of whether, e.g., a PID is 16 or 32 bits (or even longer).

The reason is to allow nasty historical implementations to still be conformant. Suppose your historical implementation had (rather common):

short getpid(void);

Of course modern systems want pids to be at least 32-bit, but if the standard mandated:

int getpid(void);

then all historical implementations that had used short would become non-conformant. This was deemed unacceptable, so pid_t was created and the implementation was allowed to define pid_t whichever way it prefers.

Note that you are by no means obligated to use pid_t in your own code as long as you use a type that's large enough to store any pid (intmax_t for example would work just fine). The only reason pid_t needs to exist is for the standard to define getpid, waitpid, etc. in terms of it.

On different platforms and operating systems, different types (pid_t for example) might be 32 bits (unsigned int) on a 32-bit machine or 64 bits (unsigned long) on a 64-bit machine. Or, for some other reason, an operating system might choose to have a different size. Additionally, it makes it clear when reading the code that this variable represents an "object", rather than just an arbitrary number.