There are several differences between the two systems. It might be helpful to think of the first year of American colleges as weakly like the college preparatory courses in France. I say that because there is some attempt to even out the pretty substantial disparities among state educational systems. College in the US is one year longer than college in France.
For example, my cousin was from Louisiana when he went to college. Because they require both French and English, they do not require students to take four years of English. That can be an insurmountable barrier to a college that requires a student to have four years of English to be admitted unless they voluntarily took extra English. Some community colleges use that first year to teach deficiencies, although these are often "zero credit" courses.
Also, most, if not all colleges have a mission. It is usually a population they are designed to serve, but not always. We have local community colleges and tribal colleges that are designed to serve the college educational needs of the local population. They are a good choice for many students because they are adapted to the local education systems. They transition students from the local public school system to that which is expected from a person with a degree.
In addition, we have a system of private colleges and universities. They often have a different mission than the public colleges and universities. Quite often, they are as concerned with the intellectual and moral formation of the student as a reasoning adult as they are with conveying content. There are also private technical schools that provide narrow education and are focused on a single professional group such as engineers.
Intermixed with these are the research institutions. They are usually doctoral granting colleges and universities. They can be extremely rigorous and selective institutions or they can have a community college component where they transition students from the local educational system to research level education.
Also, although small in number, there are a number of military institutes and academies. They exist to train the future officer corps for both the state and national military systems. They are often elite institutions. The U.S. Naval Academy, based on post naval career outcomes, is probably the nation's number two or three school.
Finally, there are the Ivy's and the Public Ivy's. They are the place where the children of the wealthy who are at least slightly talented meet the nation's best and brightest. About a third of the students are "legacies," the children of alumni. They get in because their parents or grandparents or great grandparents got in. Admission is a combination of wealth and talent.
Another factor is the difference in the goals of the French and American educational systems. Except for technical education in American high schools, the goal is to teach a little of everything. The education is general and only focused if the student chooses to focus it.
So, to try and answer your question as to why some college textbooks are similar to high school textbooks, the books fill a need. Some students enter college with two semesters of calculus, an extra year of chemistry, an advanced English composition course, and an advanced biology course. Others are struggling with math, English, and the sciences.
America is very uneven and very unfair to be born in. Without being here, it cannot really be explained because of the size and political differences. There are places in the United States without running water, sanitation systems, or electricity. There are states that still rely on "one-room schoolhouses," where all grades are taught at the same time by one teacher. That teacher is teaching first grade and twelfth grade in the same room. There are places where the teacher does not speak the same language as the students. There are schools in the US that don't have books because they cost too much.
Sometimes in the same town, there will be a public school where students are required to turn in their science lessons in virtual reality modeling language. Some schools have robotics programs. Some high schools offer college classes in their building alongside high school courses.
American colleges are the last chance for the students who have the greatest disadvantages to be given the opportunity to catch up with the most privileged.