I don't think that's necessarily entirely true.
First, don't start playing the statistics game. For example, Cambridge has an application
ffer ratio of around 5:1, whereas Harvard's is about 13:1. On the surface, this would make it look like Harvard is much harder to get into, but in actual fact UK applications are limited to 5 unis (4 if Med Applicant), but in the USA you can apply to as many colleges as you like - it just costs more! Hence, while only the best will apply to Oxbridge if they think they're in with a real chance, lots of American students will apply to Ivy League universities, just in case, even if in reality there's next to zero chance of them actually gaining a place!
Furthermore, it's generally accepted that A Levels are at least on a par with, if not rather harder than, APs (which are the closest the American education model comes to an equivalent). The only reason American students have so many qualifications is because they're required by universities!
PSAT is irrelevant - it's a practice for the SAT, and isn't actually counted for anything. SATs and SAT IIs are comparatively much easier than A Levels, but are still required by US universities to demonstrate whatever level of ability in key areas of study. Generally speaking, academically the UK secondary model is probably more stretching than the US one, (although of course you have to choose the right subjects...), and lots of UK students studying in the US have spoken about how the first year (sometimes two years) of their university education really wasn't much harder, if any, than Sixth Form. In fact, some US unis will let UK students skip a year of their degree if they have enough academic *credits* upon entering the college.
As far as ECs are concerned, it's true, lots of Americans are much more involved outside of school than UK students, but that's because US universities traditionally place much greater emphasis on these than their UK equivalents. Here, universities such as Oxbridge are far more focused on academic achievement, whereas in the US places like Harvard, Yale etc are interested in the whole package; are you a talented musician, are you widely involved in community activities, are you a good sportsman...and because this is so engrained into the system US students are naturally more likely to go into Extra-Curriculars and take them far more seriously than some of their UK counterparts, merely because they know how important they can be when it comes to application time.
Overall, I wouldn't say it's necessarily 'harder' to get into either - both have very different requirements, and it's hard to compare standards because students aren't assessed on similar criteria.
Are you thinking about applying to the US?