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 collectively
• voting is a cheap and bounded commitment (although political donations can be arbitrarily large)
• voting affects current generations and fellow citizens more relative to future generations,
• voting invokes coalitional thinking and group loyalty/morality more
• consequences of votes, conditional on decisiveness, are revealed to a degree relatively soon
• voting justifies reading about or watching politics for political junkies and policy wonks