Why has this not been done?

• We have not always had materials strong enough to make a good cable.
• People think of a cable to Geo-stationary orbit when they think of tethers. This type of tether is not going to be practical anytime soon, so many people think tethers can not really be built and don't look into them further.
• On first glance it may not look like a practical spinning tether saves you all that much rocket velocity, so some people say, "Why bother?". But, because of the exponential nature of the rocket-equation, it is significant. Also, it should make the designing of a reusable rocket possible. This is very significant.
• There are a number of legitimate technical problems still to be worked out. In particular the rendezvous problem scares people off. I am confident that there are solutions to all the technical problems, but until it they are demonstrated to work, these problems can still scare some people.
• People who could make a decision to do this have not heard and understood this idea yet.
• Not everything worth doing has already been done.