Because is not an executable file. To run the MacOS desktop application you have to run the executable from inside of the Application package.
In case of calculator run try this:
import subprocess
subprocess.call('/Applications/Calculator.app/Contents/MacOS/Calculator')
If you want to run any application without digging around for the executable file in the package (it is not always the same name as the app), use this:
subprocess.call(["/bin/bash","-c","open /Applications/Calculator.app"])