I don't think the books were made hard intentionally.
From what I understand, in the US, the same course on, say, Introduction to Calculus, can be attended by people who are learning to be MDs, chemical engineers, accountants and mathematicians because there is no fixed curriculum and everyone can pick up that course. So the textbooks and courses need to be as accessible as possible to accommodate a wide range of aptitudes.
In Russia/USSR, at least till 90s (don't know if it's still true since there had been a movement to "westernize" education) you enrolled into a major, not a school, and the whole class had the same curriculum for the first two years with some specialization starting on the third year (the whole program is 5-6 years). So students with the same major go to the same courses, which are not shared with any other majors. In this system Introduction to Calculus for mathematicians will be very different from the course with the same name for sociologists and does not have to be as accessible for people who do not really care about strictness of proofs, structure and other "mathy" stuff.
From what I understand, in the US, the same course on, say, Introduction to Calculus, can be attended by people who are learning to be MDs, chemical engineers, accountants and mathematicians because there is no fixed curriculum and everyone can pick up that course. So the textbooks and courses need to be as accessible as possible to accommodate a wide range of aptitudes.
In Russia/USSR, at least till 90s (don't know if it's still true since there had been a movement to "westernize" education) you enrolled into a major, not a school, and the whole class had the same curriculum for the first two years with some specialization starting on the third year (the whole program is 5-6 years). So students with the same major go to the same courses, which are not shared with any other majors. In this system Introduction to Calculus for mathematicians will be very different from the course with the same name for sociologists and does not have to be as accessible for people who do not really care about strictness of proofs, structure and other "mathy" stuff.