Please allow me to explain why we say and believe that everyone that is born is been born as a Muslim our Lord informed us in His Qur'an and says, [And [mention] when your Lord took from the children of Adam - from their loins - their descendants and made them testify of themselves, [saying to them], "Am I not your Lord?" They said, "Yes, we have testified." [This] - lest you should say on the day of Resurrection, "Indeed, we were of this unaware] [7 v172] Allah gathered all the progeny of Adam before they were born and came in to this world and they were all inside of the back of Adam,Allah took them all out of the back of Adam and formed them in pairs, then made their facial features and shapes, then gave them the power of speech. When they began to speak, He took a covenant from them by asking them, (Am I not your Lord?) They answered, (Verily, You are our Rabb) Allah then stated, "I hereby make all the seven strata of heavens and earth as well as your father Adam a witness unto you lest you should employ an excuse of being unaware of it on the Day of Resurrection, just like our Lord says, [Or [lest] you say, "It was only that our fathers associated [others in worship] with Allah before, and we were but descendants after them. Then would You destroy us for what the falsifiers have done?] [7 v173] and also when we were having this conversation with our Lord He informed us where we will be residing in and that He will remind us of His signs so we can recognize just like He tells us and says, , [We said, "Go down from it, all of you. And when guidance comes to you from Me, whoever follows My guidance - there will be no fear concerning them, nor will they grieve] [2 v38] and also says, [And those who disbelieve and deny Our signs - those will be companions of the Fire; they will abide therein eternally] [2 v39] and also Allah says, ([He] who made for you the earth a bed [spread out] and the sky a ceiling and sent down from the sky, rain and brought forth thereby fruits as provision for you. So do not attribute to Allah equals while you know] [2 v22] Allah says here, [So do not attribute to Allah equals while you know] meaning every human been knows who Allah is and when they hear His name they respond without them realizing.
So you see now why we say everyone is a Muslim at birth that is because we have submitted to Allah before we came in to this world but was made to forget this conversation for a test and the freedom of choice too, that is why Allah kept on sending messengers to remind us and to reason with us through His signs that is all around us that is why Allah says, [And thus do We [explain in] detail the verses, and perhaps they will return] [7 v174] Allah wants us to remember Him through His signs and return to him as we gave our covenant to Him that we will always be submissive to Him.