Because people learn the 'oo' sound and it makes sense to them that lose is spelled as loose. This is due to a lack of understanding of English, even for apparent 'fluent' speakers. It can't even be down to some kind of short-hand since the word is actually longer with the double 'oo'. I suspect it's something to do with the fact that people don't read as much as they used to and computers have spell checkers so people are not required to make the same effort when they are learning. As a result they do not get the unique 'rules' of English properly drummed into their brain and mistakes subsequently follow.
This bugs me to see as well but my English is far from perfect so I don't complain.
A
Pieces are loose if they are en prise; sometimes a loose piece will cause you to lose. A loose game may be chaotic.
English is a terribly difficult language, especially for native speakers because we have more reasons to avoid learning other languages, and languages in general when the rest of the world learns our tongue. Even so, anyone (native speaker and those learning English as their second or eighth language) that plays chess can probably learn to spell lose and distingish it from loose. Yet, 67.89% of forum threads that deal with losing put that extra o in the title. Why?
Edit: bold added after artfizz posted a correction.