Why is 2002 Nissan Altima burning oil?

Because oil is getting into the cylinders and is being combusted along with the fuel.

Two main routes are 1) from the piston rings being worn -or- 2) from valve train wear, most likely worn valve guides.

Either way will require an engine "top" overhaul at the least. Depending on the condition of the rest of the car it may or may not be economical to do this repair.