Here we read a reason for the praise for the first time: God’s greatness is unsearchable. The praise, as well as the reason for it, is mentioned multiple times in this psalm. David gives various reasons as he composes this psalm. It is important to note that unsearchable
is something different than unknowable.
God reveals himself to our salvation, so that we (can) know him. But to know him and to fathom him, like we, as people, can know each other (to a large degree)—that is not possible. God is just too big and too lofty for that. Here, that loftiness is one of the reasons for the praise.
3 Great is the LORD, and greatly to be praised, and his greatness is unsearchable.