Because the 4-3 is the base defense with a minimum of 4, not 3, rushing on every play. It isn't a blitz until you send one or two additional. Because you play the 4-3 doesn't mean you stop blitzing but you do generate more of a pass rush and pressure each and every play with 4 versus 3. In addition, those 4 DL aren't instructed to "occupy blockers" so the LBs can make a play. They are expected to make plays.
Again, on third and long, don't the players in the nickel have the same responsibility? Isn't Ziggy supposed to be getting after the QB?