Many people don't care, they're cheap enough that they're fun.  To me, and a lot of other folks it's just a pure giggle to run these things.

I used to spend money upgrading PCs and wasting electricity on the old distributed computing projects like RC5, OGR and SETI@Home.  Those made nothing back, it was just a race on a web page.

Now I spend money of mining widgets, but at least they make a bit of money back so I can buy more mining widgets.