Why would they be?
For the same reason as vessels in domestic trade and OSVs working in US waters has to be built at US yards; to protect US yards from foreign competition, thus allowing them to over charge their customers, pay high salaries to their Executives and dividend on investment, while avoiding high cost of modernizing to stay competitive.
Surprised to see that you think it is OK for large contract in the Offshore industry, worth Billions of Dollars go to foreign companies though. (???)
A) OK it was bent frames not plates as first reported.
B) Correct. Very few if any of the Drill ships, Semi-subs or even newer Jackup rigs operating in the GoM are US built and flagged. The few that are, are built at Keppel Amfels and with mostly foreign equipment, just like the newer vessels working in the GoM.