Why do people ask questions that say "How do you" (do this) when they really mean "How can I do it"?.

Because they want to know how YOU do it so that they can adapt it to something they can do. Or just figure out if it is something they can do at all—like, if they even have a chance at doing it.

Well, it’s also a result of the general collapse of English language integrity.

In a way, the phrase seems to be a mild flattery. It recognizes that they have an ability that you lack, and is mildly self-deprecating; depending on how you say it, it often carries a note of exasperation with yourself. In short, it makes the helper feel good, as though they have something to offer and are not simply being called upon as the nearest person at hand. This is not always the case, now that the usage has become standard, but I imagine that this is at least part of its origins.

It could also be a result of the younger generation (and other internetty folk) using such a form to get answers from Google. For instance, I get more hits (and totally different results, I will note) when I search for “how do you make a pie shell” versus “how does one make a pie shell”. Extending this, many article writers use terms like “How to improve your sex life in 10 easy steps” versus “How one would improve their sex life using these 10 simple steps”.

http://dictionary.reference.com/browse/you

2. one; anyone; people in general: ‘a tiny animal you can’t even see.’

“How do you do this” means “How does one do this”, not “How does the person I am speaking to do this”.

Exactly, @MrItty. What they really mean is “How does one do this?” (equivalent to “man” in German, as “Wie macht man das?”), or “How is this done?” The way we say that colloquially in English is with “you.”

Incidentally, one of the rules on WikiAnswers is that such questions must be written with “you,” presumably to get more hits because they’re expected to be worded that way in searches.