This energy is too expensive.
Today, it costs about $50 per million BTUs to extract natural gas from undersea methane hydrate.
U.S. natural gas costs $4.2 per million BTUs.
“Today” is the key word in your post.
Natural gas was significantly more expensive just a few years ago, before large amounts of fracked natural gas became available. The price was admittedly still much less than the $50/MMBTU that you quote for extracting methane clathrate, but no one has had a real need to seriously go after them and develop efficient equipment and techniques.
Once that need arises, the energy is there.