Because they were the result of bubbles bursting, not the Fed raising interest rates. The 1981-82 recession was very steep, but the recovery was very quick, precisely because the Fed induced the recession by raising rates, and then ended it by lowering them. Our recent recessions have featured bubbles bursting, so the Fed has ended up lowering rates to try to induce recovery, although each time monetary policy has been less potent. As our mega credit bubble has burst, we are now at a point at which monetary policy has no traction - we've reached the zero bound, and are only left with more unconventional measures like buying longer term Treasuries - so we can expect a steep decline, and a slow recovery. At best we will have a U shaped recession; at worst, an L shaped depression.
