Because eternal salvation is a lot more important than 80-or-so years on earth.
This mindset is why traditional Christianity is distasteful to me. It places evangelism over compassion for your fellow man, and it paints God as an asshole; the former is dehumanizing, the latter frankly blasphemous.
And now that we’ve minimized the spread of so many diseases, the risk of spreading disease is a lot less than it once was.
Still too great for these people, it would appear, or it wouldn’t be illegal to make contact with them.