Why are Muslims connecting the testimony of God's Oneness with the messengership of Prophet Muhammad?

The meaning of connecting “There is no deity but God,” and “Muhammad is the Messenger of God,” is to elucidate the accepted manner of worship. The phrase, “There is no deity but God,” means that there is nothing in existence that deserves to be worshipped and submitted to except for God Almighty. The question that logically follows after this belief has been internalized is, “How can I worship and turn to God?” The answer is: Worship God according to the Shari’ah of our master Muhammad (peace and blessings be upon him).

There is no worship except of God, and there is no worship of God except according to the way of Our master Muhammad. The two statements together form the prerequisites for the accepted action in the sight of God. God does not accept an act unless it is sincere (meaning that it is done for His sake Alone) and correct (meaning that it is in accordance with the Shari’ah of His Prophet).

God Most High says, “Say, (O Muhammad, to mankind): If you love God, follow me; God will love you and forgive you your sins. God is Forgiving, Merciful” (Quran 3: 31) and He said, “Whoever obeys the messenger has obeyed God, and whoever turns away: We have not sent you as a warder over them” (Quran 4: 80).

The Prophet (peace and blessings be upon him) taught us that there is no acceptance of devotional acts unless they are done in the manner of his acts of worship. For this reason he said, “Pray as you have seen me pray.” He also said, “Take your acts of worship from me.”

This is a short explanation of the connection between the phrase of the Oneness of God Almighty, “There is no deity but God,” and the phrase, “Muhammad is the Messenger of God.”
And God Almighty knows best.