funny you should ask this - i have often thought the same thing and it was, in fact, one of the major reasons i began questioning the faith in which i was raised.
my family is mostly fundamental christian, and several members are missionaries. they spend much of their lives in other countries "spreading god's word". and we ain't talking about metropolitan, developed and urban areas either where they would get laughed off the metro rail.
no, they head off into the jungles with their bibles and chocolate bars and trade jesus for comfort food and soda. i didn't get it either. the way i was brought up was in the belief that the only way you were "damned" to hell was if you "rejected" christ. this was how they explained that babies and such got to heaven. (too young to have done so, you see). so it followed that those tribespeople and bushfolk were in the same category. i couldn't fathom then, why my relatives would run off and risk these people's souls and give them an opportunity to reject something when they were fine before.
they never gave me a good answer either. it was always along the lines of "well, god has called me to do it" or "everyone needs to know the love of jesus and i can't be responsible for those who turn their backs on him".
talk about cold. and making no sense either. all that did was say to me "i have someone i want you to meet. he really, really loves you, but if you don't love him back he'll punish you forever in hell. here's a candy bar."
now you know why i'm an atheist.