For users, downvotes are low cost influence without participation or accountability, and for forum owners, it is a feature to reward the long tail of the least

For users, downvotes are low cost influence without participation or accountability, and for forum owners, it is a feature to reward the long tail of the least valuable users, who together make up a huge cohort, and it keeps that cohort engaged. Good of this site to be earnest, but I'd argue there is no meaningful "why," for downvotes, it's just a way to pander to the worst part of peoples nature.