Why can I view some Unix executable files in Mac OS X and not others?
Executable files may be scripts (in which case you can read the text), or binaries (which are ELF formatted machine code). Your shell script is a script; git is an ELF binary.
You can use the file command to see more detail. For example, on my nearest Linux system:
$ file /usr/local/bin/git/usr/local/bin/git: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.18, not stripped