No, I don't think the humanities are easy. After all, my own degree was somewhat of a joint honours in both humanities and STEM.
But we're talking about the chances of admission here, in relation to pupil diversity. Assumptions of lower intelligence, devaluation etc etc are all things you have brought into the discussion. It is irrelevant. As you very well know, at its highest levels, science is art. And vice versa. Remember that philosophy is the root of all logic and scientific reasoning.
But back to facts. More competitive courses are harder to get into. Fact. So, if one merely wanted to 'go to Oxbridge', the easiest way in would be to choose a less competitive course. You can be angry, accuse me of academic snobbery or whatever, but that's a fact. And, for these universities, they are also the ones with a lower proportion of international students applying, so less scope to lose their - what is it now - triple the fee of a home student.
Schools also know this, which is why Oxbridge candidates are pre-selected and prepped not just on entrance requirements, but subject choice.
Also, the ability range of humanities and STEM is also different, in my humble opinion. For a STEM subject, the divide between the excellent and truly gifted is much, much more pronounced. I am excellent, enough to have completed graduate level maths courses. But I will never be a Ramanujan or Fields medallist.
There is no such difference in the humanities. You either have the flow, or you don't. And if you do, you're all similarly gifted, and extremely capable of applying your ability to diverse and esoteric areas of research, whether it be Asian Studies or Anthropology. Bear in mind that these are academic institutions, not college of performing arts, creative studies or whatever so we are concentrating solely on academic, rather than 'creative' ability, although they're linked.
For STEM, if Oxbridge wants to take the true geniuses, they have a much smaller pool to choose from. I doubt that a load will be even concentrated in one place anyway. But for the humanities, its much larger. The difference here becomes the wider reading and knowledge beyond the syllabus - those in the know start prepping students in their area of interest early on.
Also @TenSheds why is this relevant? Not because we are pitting one against the other. But because of the assumption that more 'diverse' pupils in Oxbridge is a worthy aim.
If more students from lower SE backgrounds go for the competitive subjects, for which many able people get rejected anyway. Compared to more well-off pupils choosing the other subjects.
Then the question really becomes about pushing people away from some subjects, towards others. Just for the 'Oxbridge' degree.