The American manufacturing process and modular design of the Sherman kept that army rolling, and very organized repair and refurbishing depots kept that maintenance work running in top gear. Albeit total air superiority in Western Europe allowed that to happen as well, the Luftwaffe wasn't bombing tank depots on a regular basis..
It's actually a bit of a myth that Russian tanks are particularly T-34s were shoddily built. They were rough, and not aesthetically perfect, but their structural welds and engines were strong exactly where they needed to be. The V12 diesel was a very robust engine.
German tanks had an excellent advantage in the defensive war they found themselves in, but over engineering particularly on the Panther and Tiger II severely limited their mobility. They were too heavy for their gearboxes, and a repair that might take a Sherman offline for a day took many days for a Panther.
You might not want to take a hit in a Sherman, but at least you had a reasonably good chance for turning and running for cover without your transmission eating itself.
Tiger I - Panzer III - Panzer IV - Chi Nu Kai - Panther G - KV-1b 756(r)
M4A3(76) Sherman - T-34/85 - M3 Grant