What you are missing is that our government has made it way to much trouble to drill on shore. Yes some drilling does happen but there are way too many people that complain about it and way too many regulations.
We still drill onshore. Far more wells are drilled each week onshore compared to offshore.
http://gis.bakerhughesdirect.com/RigCounts/default2.aspx
But offshore drilling of this level is “hunting elephants”. They spend the billions needed to bring a field like this into production with the expectation of a lot of oil.