[magila]

Because government grants demand it. Private patrons can trust their own judgment when deciding who to fund, but when it's taxpayer money being handed out people are understandably going to demand objective metrics to guard against corruption. In academia the objective metrics of choice are publication and citation count, so here we are.

[christophilus]

This is the answer, or a very large part of it. Incentives are set up such that this is an inevitable outcome.