I think we still have to keep an open mind on this stuff, the goldilocks zone might be a little too perfect. NASA has still to yet rule out if Mars and if Jupiter's Moon Europa are dead worlds. Forms of life have been found miles deep in the colds of Antarctica, found in hot thermal vents under extreme pressure at the bottom of the ocean and forms of life have been found in decaying Russian nuclear radioactive submarines.
The other thing of note is that MARS has a 64% rating of being Earth like. So is this scale logarithmic or something because what does 50% mean, their gas planets?
I would also state that Earth's orbit and stability isn't really that friendly to life either and as you said it may be possible that just because a planet is in the habitable zone it is really habitable.
The only thing meant by habitable zone is that temperatures are such that liquid water may exist. That does not mean liquid water is in fact present, that life exists there, nor that life cannot exist elsewhere. But as a beginning point, it makes a lot of sense to start here since soon we will be able to analyse the atmosphere of some of these planets. We do not have a game plan to be able to do things like look under the ice of moons outside of this solar system.