This is a common assumption, but an inaccurate one.
http://web.mit.edu/firstyear/2019/subjects/ indicates that MIT’s general institute requirements include 8 humanities, arts, and social studies courses (about a quarter of your course work), including breadth and depth requirements within those categories. That is in addition to math and science requirements that include chemistry and biology as well as math and physics.
Caltech students must complete at least 108 (22%) out of 486 credit units in humanities and social studies, according to https://www.hss.caltech.edu/content/core-courses . There are also breadth and depth sub requirements. Of course, there are also math and science requirements that include chemistry and biology. See page 222 (as labeled) of http://catalog.caltech.edu/pdf/catalog_15_16_part3.pdf .