I am sure that /u/the_chieftain_wg could expand on it in excruciating detail but the notion that US tanks were not intended to engage with enemy tanks comes from a misunderstanding of Tank Destroyer Doctrine. At no point do US field manuals describe the M4 Sherman or comparable American tanks as being inadequate at engaging with armored targets.

Instead, American tank destroyers were imagined by Lesley McNair, an artillery man by trade, as fast moving vehicles that sacrificed defensive fixtures like closed turrets to emphasize big guns and speed, and then concentrated into dedicated tank destroyer battalions (along with towed AT weapons), enabling them to move from hot spot to hot spot engaging with enemy armor as they broke friendly lines. While nice and clean on paper, tank destroyer doctrine was rarely ever employed.

That Tank Destroyer Doctrine combined with some misnomers- McNair felt that tanks were for exploiting breakthroughs and supporting infantry, which leads some to misconstrue the M4 Sherman as an infantry support tank which is a separate concept entirely. Infantry Tanks were a design concept from the interwar period the French and British and Germans (early model Panzer 4's were envisioned as supporting infantry) which understood that infantry support necessitated slow, well armored tanks that were typically equipped with some form of artillery piece (as opposed to high velocity guns firing solid shot) to disrupt and break up dug in positions.

It's an easy mistake to make considering that Shermans were well armored- Jumbo Sherman's were as heavily armored as German heavy tanks- and sported a 75mm gun which traced it's lineage back to a French field gun. The problem is that the Sherman was also reasonably fast on it's feet and it's gun was perfectly capable of dealing with contemporary armored threats- the M3 75mm gun could punch through about 60mm of armor from 900 meters. This was enough to deal with Panzer 3's and 4's that were common place in Africa and Italy.

In reality the tank that most closely resembles an infantry support tank in the US army was an M4 Sherman variant that was equipped with a 105mm howitzer along with a variant of the M5 stuart- the Scott.

That, and popular mythos. People really take Kelly's Heroes to heart along with books like Death Traps that go out of their way to attempt to portray the M4 Sherman as, well, a death trap. A very easy way to explain why the M4 Sherman was so terrible is to explain that it's 75mm gun was never intended to combat tanks- those are for fighting something else entirely, after all!- and that 76mm equipped tank destroyers were intended for that role. M4 Shermans just found themselves in the unenviable position of doing a job they weren't intended for, the logic goes.