You may loose your job at start up at the most unfortunate moment. In 2000 you can find 10 jobs in about an hour, in 2001 you could not find one job if you life dependent on it. Big companies do layoffs too of cause - but probability to be cut is lower and they almost always offer generous severance package. Also your personal circumstances may make lay off extremely inconvenient - for example right before child is born or you have medical emergency.

You should definitely get some premium for this risk - how much is open to interpretation.