In my case for instance I don't NEED the money from my signature. Most people don't I reckon. Most people live comfortably without that extra income. So why do they still do it?
From what I've seen, especially at the peak of the ICO/bounty-spam, many people (in poor countries) really needed that income. I think (but never actually checked for it) most users in the CM-campaign live in countries where the money is nice, but far from enough to make a living. If you'd live in for instance Vietnam you can probably live comfortably with just a Bitcoin paying signature campaign. But if you start relying on it, you're risking your everything when the campaign ends.