For many of us, it is unpaid labor.
Strange things happen out there among the pixels and bytes.
Sometimes a site becomes too popular for the author to be able to pay the rising hosting fees, so that site vanishes. Professors also go their own ways, leaving marvelous sites behind them which crumble into memories among the .edu's . Sometimes the lives of webmasters simply change in subtle ways, or dramatic ways, leaving no time for further work, so they pull their sites out of the web. Sometimes webmasters even die, and although devoted friends may keep their sites going for a few months, or years, eventually links break, data becomes outdated, and that site dies as well. My own site, Mything Links, has been online since 13 November 1998.