Why does an apostle have to assign each missionary to a particular mission by inspiration? Why doesn’t the church just administratively assign everyone to wherever there’s a need? If you’re willing to serve, then what does it matter?
The problem with your question is that it doesn’t appropriately distinguish the meaning of “have to”. Because, reasonably speaking, they don’t “have to”. They could, of course, entirely ignore the Lord on the matter and just set about acting like the corporation that so many naysayers accuse them of being.
But this is not the Lord’s way. It is one of the core beliefs in the Church that:
“We believe that a man must be called of God, by prophecy, and by the laying on of hands by those who are in authority, to preach the Gospel and administer in the ordinances thereof.” – 5th Articles of Faith (emphasis mine)
So realistically, depending on what we mean by “have to”, we do, after all, “have to”. Or, as the Article puts it. We “must”. But as with all things in the gospel, agency could, theoretically, be exercised to go against God’s will and send people out without the use of prophecy by those in authority to preach. But if we did this, then of course we would be going out without God’s authority, or as it is put in Hebrews 5:4, we would be, inadvertently, taking this honour unto ourselves.
Let’s not confuse ourselves too much here though. Being called by prophecy (revelation) does not necessarily mean that they have to be called by the Twelve or the Prophet himself. That is the current practice. Theoretically, I can visualize, within mine or my children’s lifetime, the Church having twice the number of missionaries it does now. 160K missionaries would mean an average 1,538 new calls each week. If each missionary assignment takes only thirty seconds, that’s just shy of 13 apostle-hours per week; and I think there’s a very good chance that under such circumstances the Church would start doing missionary assignments in some other way. But whatever way that might entail, it would still be by the spirit of prophecy, and not by the spirit of bureaucracy.
I’ve heard so many stories that were just amazing as to the places that missionaries are assigned. I personally know of one that was sent to the Eugene, Oregon, mission and due to that has been able to teach his mother’s biological family. She was adopted out.
Or the sister missionary that was sent to the same mission that her dad went to and was actually able to teach some people her dad had taught but was transferred before they got baptized.
I think there is so much inspiration into where missionaries go that basic information on their paperwork just doesn’t show.
Here are two other experiences I have heard that I think are neat regarding inspiration:
1) A couple decided to serve and said they did not want to serve in a particular area. The reason, her son went missing and they never heard from him and did not know if he was alive or dead. They received the call, and she was called to where her son went missing. They declined and didn’t serve. They attempted again, and were called to the same mission. They accepted. They served in a prison. At one point, as she didn’t know the language she said in English, “Why is this at all important, why am I here. Why does it matter?” One of the prisoners said, “It matters to me.” This prisoner ended up being her son who had ignorantly broken a law he was unfamiliar with and was placed in jail.
2) A story of a couple who had specified they did not want to serve a proselyting mission. They were called to a proselyting mission. My bishop explained that the first street they chose to knock (even the first home), was the home of their son whom they hadn’t seen in 20 years (they had no clue either where he was), and who had fallen away from the Church. He didn’t specify the outcome, only that they were reunited with their son.
Now, to be clear, I don’t think these stories are common, or even necessarily the core reason why we must be called of prophecy. But they do show that the Lord’s hand is involved, and we need to trust in Him.
This is the Lord’s work. He is in charge. He sends us to serve where He will have us serve.
Of this I testify.
P.S. Here’s an interesting recent Church article and a couple of good photos that discuss the process a bit.