Early production Lee tank and Sherman tank.
In 1940 the Germans invaded France and one of the strongest tanks in their army was the Panzer IV with the short barreled 75mm cannon. The US Army had sent observers to France to report back on the action and the 75mm guns were a big surprise to the US Army. American medium tanks only had 37mm guns so something had to be done. The American M2 and M2A1 medium tanks with 37mm guns in a fully rotating turret had been outclassed overnight. Over 1,000 were on order but were now obsolete. America needed a 75mm gun tank and right away.
There had been a proposal to mount a 75mm gun in a sponson on one side. They had the blueprints and even made a mock up. It would take at least a year to design and draw the blueprints for a tank with a 75mm gun in a fully rotating turret. So they modified the self-propelled gun version into a tank with a 75mm gun in a sponson and called it the M3 Lee tank. It was intended only for training so the troops could get used to a 75mm gun while they waited for the production of the new tank.
In September the first prototype tank was completed with a fully rotating turret armed with a 75mm gun in American history, the T6. After a few modifications it was standardized as the M4 Sherman tank. So don't compare the Sherman to the Panther or the Tiger, they were years newer than the Sherman, compare it to the Panzer IV, the tank it was designed against.