I really hope nobody with any industry experience thought SO could be cloned in a weekend. That notion screams "The longest program I have ever written was for a CS lab, and I got extra credit for it because I am the awesome."
But, hypothetically supposing that it had been cloned in a weekend, then you'd be staring at the 90% of a software business which does not happen in an IDE. My app can be cloned in a weekend. It has been before, by someone who was PO'ed at me and wanted to drive me out of business. He did a pretty good job in his weekend, too -- I think the user experience of the app was probably better than I managed for 1.0, which took me a week and change.
But the other 90%, yeah, that has taken me about 3 years and a week now. It has had a lot of mental effort, sweat equity, and failing-my-way-forward invested in it. And the business, as distinct from the app, is fairly resistant to cloning.
Somewhere out there, in the forgotten corners of the Internet where not even Googlebot dares to tread, the earstwhile weekend app is probably still wondering what went wrong.