Cause humans works that way. We don't calculate risks based on statistics in everyday life. That means you can't just make people aware, you need to build cultural norm around that. But cultural norms without enforcement are easy to erode. When norm is systematically broken it ceases to be norm.

>seatbelt enforcement isn't necessary because everyone voluntarily wears their seatbelt

You obviously don't travel much. There are whole countries where seatbelts are rarely used, and drivers buy special plugs for belt receptacles to silence beeping.