Venezuelan Crude is very Heavy. They have to mix it with better quality Light Crude to make it marketable.
They have vast Reserves, but by itself it is very difficult to Refine.
They need to Import the better quality Crude to make their own Product usable. No Money, no Imported Light Crude.
Heavy, and sour too (high sulfur content). They could let their customers be concerned about mixing their product, but their own refineries need it too.
FReepers have commented that Venezuela, as a retrograde 3rd world country, has little or no refining capacity.
Common sense would have been to invest in refining capacity back when they had mucho $$$ rolling in.