The answer is simple, because Jesus sent them out in pairs. Certainly a part of this had to do with safety, and companionship, but there is another reason to send them in pairs. By sending them in twos the truth could be better protected.
We human beings are funny creatures. Let to our own devices, the stories we tell change over time. Even "eyewitness accounts" which is what the apostles provided change over time. We tend to highlight and expand the parts we like or the parts that make us look good, we tend to diminish the less flattering parts. History slowly mutates into legend.
Sending the apostles out assured two points of view. But more importantly them could keep each other grounded in the truth. There would always be some there to curb the embellishment and remind them not to leave out the not so pretty parts.
When we read the gospels today, part of what points to the veracity of the texts is that the apostles are not painted in the most flattering light. We see their imperfections. They are not mythic heroes. They are flesh and blood human beings struggling to understand.
We have not one gospel but four. As these eyewitness (apostles) died off, the four evangelists like the four points of a compass, committed to writing and handed on to us in writing the story of Jesus as seen from four points of view.
To the best of our knowledge St. Matthias was stoned to death in the area we now call the country of Georgia.
St. Matthias, pray for us.