Because all of this take place on ocean floors over hundreds of millions of years.
I agree and firmly believe that the life cycles in the oceans, including plankton and higher life forms, involves everything falling to the sea bottom when it dies, and being processed by microbes, heat, and pressure into the black ooze.
I'm not so sure it takes hundreds of millions of years however. The pressure at the bottom of the ocean could very likely be able to force the oil to other locations including caverns or porous layers under dry land. This especially makes sense if our pumping oil creates a fluid imbalance and "new" oil seeps in to replace it.