There's actually a good reason that you get credit for a long time and that is successes are so rare and valuable that you deserve credit for a long time.

The vast majority of these silicon-valley companies are built with investors' money (typically from Venture Capital firms). The VC business model generates outsized returns on a few successful companies which pay for all the failures. Careers and entire funds can swing to success based on the outcome of one company so if you're a founder you deserve that credit. It also means you've returned tons of extra money to investors that pays for all your subsequent losses.

Kevin Rose specifically is an interesting case because Digg was not a success for his investors, but her personally did well.