I think it's just prohibitively expensive at the moment. 100km (I think it would have to be a lot more anyway) is not much in terms of space exploration. And even then you will always use the majority of your fuel for acceleration towards and deceleration at your destination (however fuel needs for deceleration might be much lower if you can do some sort of atmospheric braking).
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You would have made the first 100km of the journey more fuel efficient, is that not a good enough reason?
I think it's just prohibitively expensive at the moment. 100km (I think it would have to be a lot more anyway) is not much in terms of space exploration. And even then you will always use the majority of your fuel for acceleration towards and deceleration at your destination (however fuel needs for deceleration might be much lower if you can do some sort of atmospheric braking).