Decoupling isn't the same as statelessness. The state machine pattern used by OpenGL is a logical decoupling to the physical state of GPU hardware. State machines, buffers, protocols and the like - abstractions that we tend to think of as "low level" or "implementation detail" - are in fact the primary means of decoupling used when dealing with stateful systems. Computing hardware itself makes considerable use of these abstractions in providing modular interfaces.

Stateless abstractions are great when you can provide them. But the stuff you're building on isn't itself built out of statelessness.