I think you may be confused about the nature of the problem the skycrane is trying to solve (however I also may be confused!). The rover, being a rover, does not need to stay at the landing site. If the landing site has a lot of dust blown off it, well, they can go somewhere else where the dust has not been blown off the surface. As the other answer says there may even be advantages to having the dust blown off the site as it may reveal interesting underlying geology.
However it is quite desirable that the rover should survive the landing, ideally without significant damage. And it is also desirable that it should be able to leave the landing site.
As is visible from the landing videos, the engines on the skycrane raise a fair bit of dust on the surface. They're doing this from a distance of at least 7 metres (the cables are 7m long). Imagine what they'd be doing from a metre or less above the surface (the exhaust velocity of the engines is significantly supersonic): they'd be digging large holes in the surface and picking up stones and throwing them in all directions. This would almost certainly damage or destroy the rover and surround it, probably, with significant holes in the surface which would make leaving hard or impossible. Curiosity, in fact, was damaged on landing: one of its wind sensors was damaged, they think by stones picked by the skycrane engine plumes.
So the skycrane makes the difference between a landing which is almost certainly survivable and one which is almost certainly not.
As to why the cables aren't longer: well, longer would probably be better in terms of possible damage, but would also involve carrying more cables to Mars which isn't cheap, and I think would also mean the rover would swing about more which would be problematic. There will be engineering tradeoffs here that they have thought hard about.
Here is some information from NASA, from before Curiosity's landing, which helps to explain how they were thinking:
"With a payload this size, the rockets could kick up enough dust to compromise the rover and its instruments," explains Sell. "And the rockets could excavate craters Curiosity would have to avoid as it drives away. Add to that the risk of a big, heavy vehicle driving down off the lander via an exit ramp to reach the surface."
I believe that Steve Sell (who is the person quoted above) leads the team that came up with the skycrane.