So many people with the republican/ democratic.?

why do so many people (not everyone) blindly vote for someone based on whether they'redemocratic or republican? Alot of people do not care about the issues and bicker on about how the other sides horrible. They believe everything the person on their side says but believes the other side is lieing right through their teeth. I think its ignorant. I think people should be open minded about who they will vote for and not know which side they will vote for before they even know who the candidate is...