God is creative...
It doesn't take an expert in some field to recognize that our world around us is brilliant in both its design and creativity. Part of God's design and plan was to create people groups (clusters of people who speak the same language, share the same history, geography, religious traditions, and more) of amazing diversity. These people groups would be different but they would all have within them the same longing for the eternal.
Today, there are thousands of these people groups scattered all over planet Earth. Some of these people groups have been deeply penetrated by the gospel of Jesus. Other groups know nothing of Him or His ways.
Jesus promised that someday some from every tribe and people and language would stand before the throne in worship (see Revelation 5:9 and 7:9 as good examples). In Revelation 21:26 there is an amazing picture of "the glory and honor of the nations" being brought into the Kingdom of Jesus. Like the Olympic opening ceremony we witness every four years, a procession of the nations (not political countries) will come one day before Jesus bringing Him the glory and honor of their people!
This video produced by Pioneers explains more fully about the nations and about unreached people groups: