Among other reasons, because :

• they believe the odds for voting (voting and polling data are more solid evidence)
• it's socially popular, and voting can be good for your social standing; people tend to live in politically segregated localities and communities, so for most people voting means affiliating with the groups your associates are connected with
• there are huge marketing campaigns for voting since politicians and advocates have a vested interest in convincing people to vote for them, and in the process produce generic pro-voting spillovers collectiv