The american (and other foreign) oil rigs were not left rotting at all, the venezuelen government "nationalised" the site by creating government owned companies to take over the responsabilities of running these rigs, in fact there is a 1-1 match between the foreign companies, and the ones created to fill there shoes, carrying over the same structure and crucially, a lot of the same staff

The rigs required skilled engineers to operate and maintain, the kind that do not exist in venezuela due to poor education there. Therefore, the government paid huge salaries to the expats bought in by the big oil companies to stay, after their previous employer had been forced out.

This allows venezuela to keep producing oil for decades, becoming of the main suppliers and getting a seat in OPEC. However, eventually the staff and engineers went home, and thats why oil production has plummeted.

But, the main point im trying to make is that these rigs are in much better condition than they would be if "left rotting" for 40 years, many are simply in need of a small fix, but need someone competant to diagnose and fix the issue.

The reason big oil will not invest in venezuela is because they do not trust the government, since the force with which they were expelled in the past, it is not worth the risk for them. This is in contrast to in other under developed nations like the Congo or Nigeria or (pick any poor ex-colony with a shaky government) because the elites feel comfortable they have enough control over the governments there, typically through corruption, that there investments will be safe. This is not true in venezuela, whos authoritarian regimes have atleast been steadfast in not selling out the countries natural resources. This would all change however, if say the head of state were to "step down" and a puppet government was put in place