Here's why:

* Formation: Crude oil forms over millions of years from the remains of ancient marine organisms (like plankton) that were buried under layers of sediment.

* Sedimentary Rocks: These layers of sediment eventually become sedimentary rocks like sandstone, limestone, and shale.

* Porous Rocks: The porous nature of these rocks, along with the presence of impermeable layers (like clay or shale) above and below, traps the oil.

So, while the Earth's crust is the outermost layer, it's the sedimentary layer within the crust that houses the oil deposits.