A rhetorical response question would be, "Why would one think Act 2:36 is referring to Jesus being 'made Lord and Christ after the resurrection'?" This idea is reading more into Act 2:36 than is there.
The ESV, and most translations, make the aorist indicative ἐποίησεν into "has made" (a perfective idea, a completed action). That is an interpretative move, one allowed for with the aorist tense given that it does not itself reflect timing;1 and as will be seen, it is a reasonable move.
However, notice that even if one more directly translates it as a simple aorist idea, "made," the statement does not indicate in any way the timing of this action other than as a past action, given that it is in the indicative mood. That is, Act 2:36 simply makes a direct, factual statement that God made Jesus Lord and Christ—it indicates nothing about the timing of when this was done other than in the past at some point.
Now in Luke 2:11 an angel makes a statement that does include a point of time, σήμερον ("this day" or "today"). However, the timing is about the birth of a σωτήρ ("Savior"), which Savior is identified as ὅς ἐστιν χριστὸς κύριος ("who is Christ [the] Lord." Hence, He is not just any Savior, but the Savior spoken about in the prophecies of Hebrew Scriptures, the Messiah Who was to come, Who is the Lord, has "today" come in flesh. The verb ἐστιν is present indicative. The choice of tense and mood indicates at the time His birth, "Jesus" (Luke 2:21; which is Ἰησοῦς, the Greek of the Hebrew יֵשׁוּעַ [Jeshua, i.e. Joshua], which means "YHWH is Savior," so tying to v.11 stating Savior = the Lord) is already considered as Christ the Lord.2
Thus, the Luke 2:11 passage is partly the background as to why the aorist of Act 2:36 often gets translated as the perfective "has made," because it has been revealed that He was already Christ and Lord at His birth.
Luke 2:11 still does not indicate timing of when God designated this newborn to be Lord and Christ, but it does indicate He already was so at His birth. Nothing indicates this status is lost through His death and then regained in resurrection—rather, the passage in Acts simply indicates that this One Who was made Lord and Christ is now being exalted (v.33), and it is because of His resurrection (v.31-33) that Israel should without question "know for certain" (and hence, believe) that He is the One designated as Lord and Christ.
1 The most common use of the aorist is simply as Daniel Wallace labels it, "Constative," which is the idea where the writer "views the action as a whole, taking no interest in the internal workings of the action" (Greek Grammar Beyond the Basics - Exegetical Syntax of the New Testament [Zondervan Publishing House, 1999], 557). By "internal workings" is meant how the action comes about: its exact timing, the processes needed to do it, etc.
2 The present indicative's most basic use is to simply state the current, continuing state of affairs (ibid., 514ff.).