There's a few reasons:

Smaller class sizes for general education courses so the teacher knows you better and so do your classmates. Leads to more accountability for completing course work as you can't just be a face in a 500 seat lecture pit.

Many of the teachers coincidentally teach advanced high school courses and moonlight as adjunct professors, so that also contributes to the "high school feel".

American Universities are an institution where you are expected to act professional and complete coursework as per the guidelines. There is no one holding your hand to complete the coursework. Thats where you have a lack of rigidness. No one cares what you do once the semester begins. You can omit attending classes and accept the consequences of your actions. You are an independent adult and you are treated as such. Of course many schools offer numerous resources to get help in the coursework but the person must take the initiative to keep up if they are struggling.

The schools are generally upholding a reputation (Especially for the Engineering university I went to) and with the declining standard of high schools, many college bound students are just not ready for the culture shock of a institution that expects a certain level of academic discipline and rigor and will not be kind to you if you do not meet it.(To be clear, at many schools you are still given opportunities to rectify the situation before they permanently close the door on you but it will still stain your record at that point).

Community college is open enrollment, even if you completely fail all your courses and you have a record on your transcript, there is always an opportunity to come back and rectify the failures of the past. In fact the colleges are designed to allow anyone to continually try and improve themselves academically. This allows students to raise the level of standards needed for post high school academic work on their own terms and on their own timeline. One way they do this is create a structure where you are given more attention and the coursework is more "guided" to help you get to the finish line and build that discipline that you may not have gotten in high school.

I'll give you an anecdote I got from the dean of the math department at my local community college which will help paint a picture of the situation(this was 15+ years ago so things may have changed).

The college runs a super basic Algebra math class for people who didn't complete high school or are so far out of school that they need to start at the very basics. They go out of their way to accommodate the students as this course is typically the entry into a majority of all degrees (STEM or otherwise).

To that end, they run multiple copies ("sections") of the course at all days of the week, at essentially any available time to accommodate any possible schedule. This involves hiring over a dozen instructors just for this one course but it is the mission of the school to educate as many people as possible so it makes sense. This is in addition to a dedicated tutoring center open to all students. Yet despite all of this, over half the class(I think it was 60%) failed or didn't make it to the end every semester. The community college makes it possible for students to get a second, third, fourth chance at success. As long as you are alive, it is never too late.

Even at 60% failure rate, that still means 40% of students continue on. Given enough time the hope is that the majority of the population is on the path to achieving their dreams. At least thats the hope and the Community College does everything it can to help students get there.