Because nature doesn’t always behave the same in a lab, test tube or computer program as it does in the real world, scientists and engineers have come up with ideas that didn’t turn out as expected.
Do you ever wonder how is it that so many in government and in the corporate uber-state that pervades every aspect of our planet like an invasive organism, how these people can manage not to care about climate change? You know they aren't stupid or uneducated and that they are aware of the scientific evidence and even if they are greedy , sheer self preservation demands that they deal with the problem... but they don't.
I think that is because deep down they expect that some massive geoengineering scheme will pull the rabbit out of the hat and presto!!! Everything is all better and see we didn't have to get off fossil fuel at all. We can now go on just like we have been doing.
The rationalizations used by the greedy to excuse their anti-social behavior don't bear up well under scrutiny... but then the greedy rarely bother scrutinizing their excuses anyway.
When things get too much to bear (ever hear rightwing deniers deny climate change and then abruptly admit it is real when they suddenly switch to saying it'll be too late anyway?)...the deniers - the climate skeptics - the wait until they have no choicers who then say it is already too late - will demand geoengineering be implemented.
That is when everybody takes a very big gamble with very little to go on except desperation. That looks like the big plan folks... do nothing or do as little as possible and then ... the big finale!!!
Ah... do we love the big ending moment to our movies. It'll all be fixed and we will all be saved by some near magical solution by geoengineering! See no need to worry or stop using oil and coal... we will just reduce sunlight across the whole planet...what could go wrong with that?