A big portion of that readiness was the fact you'd have to tear half a Tiger or Panther apart to get at the engine or transmission. Want to swap the tranny in the Sherman, get a wrench, pop 17 bolts, swap, and done. Same for the engine in the back. The Americans also had the extra industrial capacity to provide huge amounts of spares, compared to the Germans who were building entire tanks out of nearly everything they had rather than provide replacement parts.
Tactically, in a perfect setting, the Tiger's a fantastic piece of hardware. Purely on armor vs armor, and gun vs gun, it wins. It's also 15-20 tons heavier than the M4, so that's an obvious result anyway. Apply the situational problems the Germans had with lack of spares, bad designs (interleaved wheels, looking at you), terrible gas mileage (68 miles offroad per tank of fuel) and poor supply situations and the Tiger becomes at best a bunker with a pretty solid gun aaaand that's it.
The King Tiger/Tiger II? Add another fifteen tons to the Tiger. Keep the engine. Enjoy your 10-12mph speed cross-country because of it, not to mention terrible turret rotation speed, and the simple fact that all the armor's on the front...which led to embarrassing moments like this:
"During the Battle of St. Vith a M8 Greyhound armoured car destroyed a Tiger II after getting in behind it on the Schonberg Road, though the commander tried to traverse his turret to engage the M8. The M8 fired three 37 mm rounds into the relatively thin rear armor of the Tiger from only 25 yd (23 m), setting it on fire: "There was a muffled explosion, followed by flames which billowed out of the turret and engine port."
Ballsiest Greyhound crew ever.