Why the hell does oh-my-zsh have an .sh extension and not .zsh?

oh-my-zsh.zsh

@@ -0,0 +1,41 @@
+# Check for updates on initial load...
+if [ "$DISABLE_AUTO_UPDATE" != "true" ]
+then
+  /usr/bin/env ZSH=$ZSH zsh $ZSH/tools/check_for_upgrade.sh
+fi
+
+# Initializes Oh My Zsh
+
+# add a function path
+fpath=($ZSH/functions $ZSH/completions $fpath)
+
+# Load all of the config files in ~/oh-my-zsh that end in .zsh
+# TIP: Add files you don't want in git to .gitignore
+for config_file ($ZSH/lib/*.zsh) source $config_file
+
+# Add all defined plugins to fpath
+plugin=${plugin:=()}
+for plugin ($plugins) fpath=($ZSH/plugins/$plugin $fpath)
+
+# Load and run compinit
+autoload -U compinit
+compinit -i
+
+# Set ZSH_CUSTOM to the path where your custom config files
+# and plugins exists, or else we will use the default custom/
+if [ "$ZSH_CUSTOM" = ""  ]
+then
+    ZSH_CUSTOM="$ZSH/custom"
+fi
+
+# Load all of the plugins that were defined in ~/.zshrc
+for plugin ($plugins); do
+  if [ -f $ZSH_CUSTOM/plugins/$plugin/$plugin.plugin.zsh ]; then
+    source $ZSH_CUSTOM/plugins/$plugin/$plugin.plugin.zsh
+  elif [ -f $ZSH/plugins/$plugin/$plugin.plugin.zsh ]; then
+    source $ZSH/plugins/$plugin/$plugin.plugin.zsh
+  fi
+done
+
+# Load all of your custom configurations from custom/
+for config_file ($ZSH_CUSTOM/*.zsh) source $config_file