Are there any reasons or benefits for the very long process of reviewing grant proposal?
The processes for grants and papers are very different. Once the editor or programming committee has reached a decision, there’s not much left to do. 

Grants are much tougher. After the decision by the panel has been reached, there are several additional levels of bureaucratic approval that must be secured before a grant can be formally awarded. It’s these latter stages that lead to the extended time between submission and decision.