It's the intersection of dramatic potential effects on society with skills that HN readers have and understand.
A sibling comment pointed out the loss of life issue. I recall (correctly, I hope) Sebastian Thrun mentioning that a traffic accident was part of his initial motivation. Reducing loss of life due to human folly is a strong motivator, but there are certainly ample opportunities for that beyond just this one.
Self-driving cars have widespread potential effects across society, from shipping to taxis to car ownership to the human angle in hours saved and lives saved. This is big. Think of all the lives lost and hours wasted in traffic in the US every year. (No functional public transit in most of the country, etc.)
It's an area where the challenges are largely technical--once the technology is safer than human drivers, we assume the regulatory issues will go away quickly. (And we probably underestimate the technical issues in getting there.)
The huge potential combined with massive and primarily technical challenges makes this probably the biggest thing since the Internet where a bunch of engineers feel like we can change society in a profound way with a bit of software.
It's partly because the tech is actually so new that we can project our expectations onto it rather than focusing on what is actually here today (which is impressive but far from the goal). Weigh this potential against the reality of what most of us actually work on today, and you may see the appeal.
Of course, reality will take time to catch up to the dream, but it's the dream that generates the excitement.