If the end goal is the preaching of the gospel to every human on the planet, then it shouldn't be too hard to come up with very efficient and effective methods to do so. For example, God could send hundreds of millions of angels to preach the gospel around the world, and in a matter of minutes every person would be evangelized. Or God could give dreams and visions to every person simultaneously, revealing to them the truth of the gospel, causing unprecedented large scale conversions all over the globe (by the way, there is anecdotal evidence that this appears to be happening to Muslims to a lesser degree). And likewise, one could creatively think of myriads of other possible ways in which the gospel could've been preached sooner, faster or more effectively. Yet, God has decided to use human vessels as His main instruments to carry out the great commission. Why?
Since I don't intend this question to be opinion-based, I would prefer answers that draw on reputable sources pertaining to the subject matter, e.g., the works of reputable theologians. Alternatively, if a denomination has an official denominational answer to the question, I would be interested in knowing about it too.