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The reason for is just how damn good all the Physics Theory is.> (majewski's comment below) I think you're seeing this in physics because, in terms of experimental accessibility, the low-hanging fruit are mostly picked.
I mean, yes and no. The Standard Model is amazing. But I think we're also kind of focused too much on the Big Questions, and in the arena of the not-so-big questions there are still a ton of gaps and missing insights.
Perhaps in the not-so-big questions, most of the low-hanging fruit may appear to be picked as well, but I suspect that is more a kind of myopia that can be solved by collaborating outside of our field.
Whenever I see an article these days about physics that really excites me, it tends to be a story where one or more physicists got excited about a seemingly small question, or ate a bit of humble pie and joined forces with chemists, biologists or some other field, and or both, and that working it out turned out to bring far-reaching consequences and novel insights. Same with maths, really.
I'm aware this is completely personal of course, and that there is a likely bias in that this kind of research tends to be more accessible and thus more fun to read for me, but still.