Not sure bout you but I see this as an _extremely_ common misspelling on the web. I would love to know how it arises. Phonetics? Soundalike? Lose and loose certainly have very different meanings.
Maybe there's a similar word in another language. There's lot of mistakes of this kind. For example, french often say "actually" when they mean "currently", because its translation is "actuellement".
We could probably get the answer by hanging out on an ESL forum for a bit, that's assuming it's non-native English speakers who are making this mistake (it might not be!).
I suspect it's almost entirely native English speakers making this mistake. Those who learn their English as an acquired language are taught about this sort of thing, and it's known intellectually.
Native English speakers just rattle stuff out, and don't know any explicit rules (and their exceptions) to go back, re-read, and check.
"lose" doesn't sound like "pose"...