Why does GCC create a shared object instead of an executable binary according to file?

What am I doing wrong?

Nothing.

It sounds like your GCC is configured to build -pie binaries by default. These binaries really are shared libraries (of type ET_DYN), except they run just like a normal executable would.

So your should just run your binary, and (if it works) not worry about it.

Or you could link your binary with gcc -no-pie ... and that should produce a non-PIE executable of type ET_EXEC, for which file will say ELF 64-bit LSB executable.