If the end goal is the preaching of the gospel to every human on the planet, then it would not be 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 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 some 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 chose to use human beings, the Church, to carry out the great commission. Why?
I don't want this question to be opinion-based, so 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 that too.