Part of it is due to the incredibly restrictive emissions in place during the late 70s/80s/90s - they have much tighter 'smog' controls over there than we do and consequently the engines are tuned, and have a vast amount of emissions systems, to meet these regulations - reducing power.
For example, you could have had a 1970 Firebird with a 355BHP Ram-Air engine (top spec), yet by 1975 the top spec engine was a mere 290BHP.
Come 1977, the top-end engine offered a mere 200BHP - all due to restrictions imposed upon them by the government.
On the flipside, however, these engines would still make an easy 300-400ft.lb, with massive spreads of torque across the rev range, and would also run indefinitely without issues - another reason for the low outputs, so the engines were massively understressed and would clock up hundreds of thousands of miles without issue or care - as the market required. These are engines that were just designed to be used, abused, and soldier on indefinitely without anything being done to them.
Many cars were specced with automatic transmissions too, where torque and low RPM ability makes for a much more pleasant and flexible drive - again another reason for the 'low RPM, high torque' stereotypes.
They were, as a result of the supportive aftermarket, also ridiculously easy to tune and with a change of intake, carb and sometimes heads, the engine could be boosted easily back up to the 300/350BHP mark - and further, without much effort.
As another example - my old L98 engined Corvettes would make 'just' 245BHP and 340ft.lb from 5.7 litres - whilst still returning sensible economy - yet are strangled by various emissions systems and 3 (!) very restrictive catalytic convertors, in order to meet the regs. Still do 60 in under 6 seconds though
Don't get me wrong though - they could still make powerful engines all through those periods, but they were either very special order, or very rare - ZL1 Camaro from 1969 for example, with the all-alloy ZL1 engine making 430BHP and 450ft.lb, or the ZR1 Corvette making 375BHP in the late 80s at up to 8000RPM (factory limited).
They could always make engines that revved too - not uncommon for some 8 litre (!) Hemi V8s in the 60s NASCAR series to be hitting 10,000RPM (!) at which point they apparently sounded "a bit funny"
You can't take these things just on face value (i.e. the classic "man, that's a poor BHP/litre/ci/etc..."), or base opinions on the typically misquoted or misuderstood 'facts' from other people
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