The grade of the route is the median of the opinions from people who have ticked it.
The default option when ticking a route is to agree with the current grade. Another options would be to "force" people to choose a grade, or then leave them with "no opinion" option. There's few reasons why the first option is the current system.
The goal is to make ticking routes as easy as possible, and that's why the current grade is chosen by default. So that the users wouldn't need to click so many times when ticking a route if they agree with the current grade. Also, it's still easy to give the opinion if you disagree.
The reason for why "no opinion" is not the default option, is because then very likely only those climbers who disagree with the grade would give their opinion. This would lead to crooked and easily changing gradings.