In order to understand why scholars turn to the use of the first-person pronoun “we” in the Acts of the Apostles to determine the authorship of the Gospel of Luke, you have to realize that the former is the sequel to the latter. For instance, compare the prologues to both books:
“Since many have undertaken to set down an orderly account of the events that have been fulfilled among us, just as they were handed on to us by those who from the beginning were eyewitnesses and servants of the word, I too decided, after investigating everything carefully from the very first, to write an orderly account for you, most excellent Theophilus, so that you may know the truth concerning the things about which you have been instructed.” (Luke 1:1-4 NRSV)
“In the first book, Theophilus, I wrote about all that Jesus did and taught from the beginning until the day when he was taken up to heaven, after giving instructions through the Holy Spirit to the apostles whom he had chosen.” (Acts 1:1-2 NRSV)
In addition to these prologues, there are solid grammatical, stylistic, and theological reasons to argue for the common authorship of these two New Testament books. I am not sure that there are any biblical scholars who deny this, though I am willing to be corrected on the matter (note: I have added one book below that does challenge the common authorship). There is some debate about whether the evangelist intentionally planned to write a two-volume work from the beginning or whether some time had passed before the evangelist got around to writing a sequel for the Gospel of Luke. There is also some debate over whether or not the author of the canonical texts of Luke and Acts had revised an earlier proto-Luke. This latter debate is tied in with the debate over the identification of Marcion’s Gospel and whether it represents either a later edited version of the canonical Gospel of Luke, or an alternate or even earlier version of Luke’s text, since Marcion was frequently charged with corrupted the text of Luke (e.g., Tertullian, Against Marcion). But this is a discussion for another day. The point here is that whoever wrote the canonical book of Acts also wrote the canonical Gospel of Luke.
Sources on the Unity of Luke-Acts
Bird, Michael F. “The Unity of Luke-Acts in Recent Discussion.” Journal for the Study of the New Testament 29.4 (2007): 425-447.
Gregory, Andrew F. and Rowe, C. Kavin, editors. Rethinking the Unity and Reception of Luke and Acts. Columbia, SC: The University of South Carolina Press, 2010.
Parsons, Mikeal and Pervo, Richard. Rethinking the Unity of Luke and Acts (Minneapolis: Fortress, 1993.
Tannehill, Robert C. The Narrative Unity of Luke-Acts: A Literary Interpretation. Vol. l: The Gospel According to Luke. Philadelphia: Fortress, 1986.
Verheyden, Josef. “The Unity of Luke-Acts.” HTS Teologiese Studies/Theological Studies 55.4 (1999): 964-979.
Sources on the Relationship of Canonical Luke-Acts and Marcion’s Gospel
BeDuhn, Jason. The First New Testament: Marcion’s Scriptural Canon. Oregon: Polebridge, 2013.
Hays, C. M. “Marcion vs. Luke: A Response to the Plädoyer of Matthias Klinghardt.” Zeitschrift für die neutestamentliche Wissenschaft 99 (2008): 213-232.
Hoffman, R. Joseph. Marcion: On the Restitution of Christianity: An Essay on the Development of Radical Paulinist Theology in the Second Century. Chicago: Scholars Press, 1984.
Klinghardt, Matthias. “Markion vs. Lukas: Plädoyer für die Wiederaufnahme eines alten Falles” New Testament Studies 52 (2006): 484-513.
Klinghardt, Matthias. “The Marcionite Gospel and the Synoptic Problem: A New Suggestion”, Novum Testamentum 50 (2008): 1-27.
Lieu, Judith. “Marcion and the Synoptic Problem” in New Studies in the Synoptic Problem. Edited by P. Foster, A. Gregory, J. S. Kloppenborg, J. Verheyden. BETL 279; Leuven: Peeters, 2011, 731-51.
Lieu, Judith. Marcion and the Making of a Heretic: God and Scripture in the Second Century. Cambridge: Cambridge University Press, 2015)
Knox, John. Marcion and the New Testament. Chicago: University of Chicago Press, 1942.
Moll, Sebastian. The Arch-Heretic Marcion. Tübingen: Mohr Siebeck, 2010.
Roth, Dieter T. The Text of Marcion’s Gospel. Leiden: Brill, 2015.
Vincent, Markus. Christ’s Resurrection in Early Christianity and the Making of the New Testament. Farnham: Ashgate, 2011.
Vincent, Markus. Marcion and the Dating of the Synoptic Gospels. Studia Patristica Supplement 2, Louven: Peters, 2014.
Von Harnack, Adolf. Marcion: The Gospel of the Alien God. Durham: Labyrinth, 1989.
Challenging the Common Authorship of the Gospel of Luke and the Book of Acts
Walters, Patricia. The Assumed Authorial Unity of Luke and Acts: A Reassessment of the Evidence. SNTSMS 145. Cambridge: Cambridge University Press, 2009.