Here Job uses a question to make a statement. Alternate translation: "I do not understand why God allows a person to keep living when the person would be very happy to be buried in the ground" (See: [[rc://en/ta/man/jit/figs-rquestion]])
The phrase "rejoices very much" means basically the same thing as "is glad." Together, the two phrases emphasize the intensity of gladness. Alternate translation: "one who is extremely happy" (See: [[rc://en/ta/man/jit/figs-doublet]])