I would think of an extremist Christian as one who sells all that he owns, gives his money to the poor, and wanders the world to preach the gospel and do good works. This was not an impossible task for John the Baptist, and he didn't even have the gospel to work with.
I don't know about St. John, but it should be noted here that the apostles still had jobs. St. Paul was a tent-maker, for instance. The idea of selling all that they own in order to go do this implies that they would've been rich in the first place, which is not necessarily the case. Some were, some weren't. Same as today.
You can disagree that such a thing is what constitutes an extremist Christian, but my question still stands: why is there no one on earth doing this?
Because it's not easy to uproot or leave your entire family so you can go to a foreign country and wander around? This is sort of the same thinking behind the restrictions on accepting novice monks in many churches, because they have to meet certain requirements that are very tough to do if you are young, still have family, etc. But it is still done anyway; if you are not seeing it it is probably because you are not looking in the right places. Just a few months ago we were blessed at my little Coptic Orthodox church to have such a monk staying with us for a little bit, who did indeed travel the country with the blessings of the bishops and HH Pope Tawadros II in order to plant communities here and also spread the word about Coptic media services in English. Not all monks live in the monasteries of the desert, I suppose, though he was the first one I had personally met who led this kind of life. He told me he just sort of wanders from place to place, and when he finds a preexisting Coptic community in an out of the way place (like ours), he'll stay a day or two, let them know what he's doing, and ask that they carry on with this kind of work after he leaves. And he was really serious about it, too, kind of getting in our faces about why we were so small, what we were doing to grow, what we should be doing, etc. I think it made everyone feel kinda bad, actually, but sometimes you need to hear that stuff.
There is also the history of the Coptic Orthodox Church in Bolivia, which started with one priest, Fr Youssef Anba Bola, who was just kind of dropped out there by the Pope himself (HH Pope Shenouda III) and told "Okay...go to work." That was in 2000, and today they have their own bishop (HG Bishop Youssef), and I have been told by people I met who worked over there that something on the order of 400+ Bolivians (no Egyptians; the church is entirely native except for the priests and the bishop, and that's only because it's new enough that they haven't raised a new generation yet to be in those positions) attend the liturgy weekly in the Coptic cathedral in the capital city of La Paz, with many more in the countryside and in other towns as the Church continues to grow there. Not sure if that fits your paradigm, OP, but it is a case of one man just kind of showing up to a new country and getting his hands dirty and building something lasting from more or less nothing. That's the way most missions are, I would imagine.