Because a project should never have platform specific code.
justin@t420:/tmp$ git clone https://github.com/tmux/tmux.git
justin@t420:/tmp$ cd tmux
justin@t420:/tmp/tmux$ wc -l osdep*
95 osdep-aix.c
88 osdep-cygwin.c
80 osdep-darwin.c
133 osdep-dragonfly.c
202 osdep-freebsd.c
41 osdep-hpux.c
98 osdep-linux.c
137 osdep-netbsd.c
157 osdep-openbsd.c
101 osdep-sunos.c
41 osdep-unknown.c
1173 total