I guess it's the same reason people start a business in any sector. They believe they can do it better or at least want a piece of the pie. I don't know if the failure rate is that much higher than other sectors. There are reports saying new restaurants fail (figures from 30% to 80% of new restaurants fail in the first 12 months). In the US, the stat is 1 in 5 companies fail in the first year and half within 5 years.
Aviation holds a special magic for a lot of people and that helps draw people in I guess. But we should also be grateful people are willing to take those risks. Sometimes it doesn't work out but still leaves us better off.