Seems like this old misconception refuses to die.
Yes, there is scarcity in some popular courses and majors, since UCs do not have Harvard-level endowments that allows them to maintain unused capacity in all departments (which would be seen as “wasted” in an environment where people complain about why their kids cannot get into a UC that is not “beneath” them like UCR or UCM). But course registration systems are designed to give priority in courses to students who most need them for their majors (and “impacted majors” are that way to avoid exceeding the department’s capacity to teach them), and give all students the opportunity to choose part of their schedules before letting any fill out the rest of their schedules (so that all students can get their most important courses first before they get filled by others choosing out-of-major electives).
Obviously, scarcity is not ideal. It also means that popular out-of-major electives are hard to get, and students trying to avoid 8am or friday class times may not be able to. But if a college manages it reasonably, that should not prevent students from graduating on time.
Of course, avoiding such scarcity may be an actual benefit to attending a super-wealthy well-endowed school with excess capacity everywhere, so that there is space in every course that one may take (but check to see that this is actually true on the school’s class schedule).