The band of "habitable by humans" is as wide as you could want. People live in Alaska and in Libya.
The problem is that stuff is adapted to the way things are. Alaskan trees, bugs, mail services and whatnot are all adapted to snow and ice and such. Bugs go dormant, mailmen hibernate. Libyan mailmen would freeze to death is they got Alaskan weather. Alaskan trees would die if they got Libyan weather. Crops would fail, etc.
The problem is that stuff is adapted to the way things are. Alaskan trees, bugs, mail services and whatnot are all adapted to snow and ice and such. Bugs go dormant, mailmen hibernate. Libyan mailmen would freeze to death is they got Alaskan weather. Alaskan trees would die if they got Libyan weather. Crops would fail, etc.