Since I work in Thailand, I will frame the question more personally for my nation. You can do the same for the unreached/unengaged nation that is on your heart.
So, why don’t the Thai people reach their own people?
The answer is that there ARE God fearing and God loving Thai people who do seek to reach their own people for Jesus, but their numbers compared to the population who don’t know God are very small, less than 1% of a population of 68 million!
Of that 1%, not all are evangelical which means not all are active in sharing their faith with others.
Many are working hard every day but they need help!
The harvest is still plentiful and the laborers still few in most places in the world outside of the West (Matt 9:37, Luke 10:2). I don’t believe that God is only using Americans to fulfill His Commission (Matt 28:19-20), but that doesn’t mean than He is excluding them from going to other nations either.
God is using many nations from all over the world to carry the name of Jesus to those who have not heard, including Americans. However most of the new missionaries that are being raised up are being sent from Asia and Africa, not the West
Whenever possible the West should partner with those native believers who need our help, but we can also go ourselves. For instance, if the Thai people have yet to establish a life giving reproducing church in a certain province of their country, nor are they planning to plant one, then I believe that it is not unreasonable to believe that God would send an Westerner, or any other nationality for that matter, who has been saying, “Here I am Lord, Send me!” in order to fill the void and/or act as a catalyst to see new works planted.
There may also be “native churches” in the land that have been planted decades ago but remain small in vision, fiercely territorial, steeped in doctrinal error, and stunted in growth.
Could God not send someone in from outside the culture to bring fresh vision and passion? Is this not what has happened recently with African preachers being sent to Europe and America to restore and revitalize dying churches? God is not limited to our linear thinking.
Sometimes, it is the “going” of the western missionary and the work that he does, that raises more awareness and subsequently more finances for the least reached places of the earth.
Some agencies would like us to believe that they have that covered for us, but who made them the one stop shop for native missionary support and projects?
I believe God will raise up “new wineskins” and fill them with “new wine.” To me, this means that each generation receives a fresh move of God’s Spirit. As young Americans and other nationalities continue to heed the call to serve cross culturally in the least reached places of the world, their service and sacrifice will be valuable for the kingdom of God and eternity.
Their work will help to fill in the gaps in needed areas and strengthen the local believers where they work.
Don’t get me wrong, if I had the choice between the effective, Spirit filled native Christian who already knew the language and customs of the people and the effective Spirit filled American missionary who had to learn these things, then of course I would choose the native.
But that’s not an option in many of the places of the world yet.
God may still be sending people across the seas with a vision to make it more of a reality than it is.
Some places are beyond needing long term outside help living among them, but most still are not.
As it has been said before but it is worth repeating,
“We need the WHOLE church to take the WHOLE Gospel to the WHOLE World.”