Why do we say God is merciful, and also ask for mercy in different parts of prayer and psalm? It’s like saying to someone you’re generous, can you be generous?

G-d is, by definition, not only merciful but also perfect and truthful. But we often don't understand why certain G-d-sent fates are merciful and for our benefit. We ask Him to please give us what's best for us in the easiest, most merciful (for us and our understanding) way. This is directly connected with our free-will and the balance of how we destine our fate, and how He harmonizes the various (13) attributes.