The answer
King David is not the only composer of the Holy Book of Psalms. Actually only seventy three of the one hundred and fifty (151 in the Septuagint) are clearly attributed to King David. Others include contemporaries of David's whom he had placed in charge of worship in Jerusalem like Ethan, Heman, and Asaph. Moses & King Solomon also wrote psalms.King David wrote (Ps 3-9;11,12-32,34-41,51-65,68-70,86,101,103,108-110,122,124,131-133,138,140-145). Moses wrote the Holy Book of Ps 90, Asaph (1 Chron 16:4-5) (Ps 50,73-83), Levites such as the Sons of Korah (Ps 42,44-49,84,85,87) Heman (Ps 88) Ethan (Ps 89), Solomon (Ps 72,127). The composers of many of the Psalms remain anonymous.
Although not all Psalms were authored by King David, the whole book was attributed to him under the general consensus that David was without doubt God's anointed one; and the ideal king who presented the psalms to the entire congregation as a way of worship.
Answered by His Eminence Metropolitan Youssef (opens in a new tab)