Why don't most people consider community colleges?

I see people that just want to go to a UCs even If it means paying a ton of money. To me, this doesn’t seem like it’s a good thing to do.
First of all, I am talking about the people who don’t get a good scholarship. Wouldn’t it better to just attend community college and then transfer to a UC.That seems like the best option since you get to save money and still get to graduate from a UC later on.

Also, How does guarantee admissions from CC to a UC work? Is it GPA based or something else…?

For your first question, some people may find starting at CC to be less suitable because:

a. They are already advanced in some subject (e.g. math or a foreign language) and will want to take upper division courses in their first or second year.
b. Their intended major’s lower division courses are not well covered by the courses offered at nearby CCs, according to http://www.assist.org (e.g. CS or EECS at UCB or CS at UCI). In such majors, a transfer student may need to spend some quarters or semesters taking “catch up” courses, which can delay graduation.
c. They prefer a four year college experience instead of two different two year college experiences.

However, starting at CC can make sense for many other students for whom the above do not apply.

Regarding your second question, see http://admission.universityofcalifornia.edu/transfer/guarantee/index.html .