Why are so many users asking questions on private messages?

Yes, particularly in years gone by, I used to get enormous numbers of "private questions", particularly on translations and languages, and partly it was my own fault, because I would spend so much time answering them. It got so I was actually spending considerably more time writing private messages than I was answering open questions. That is wrong for several reasons:

no-one else gains any benefit from the knowledge in private answers (and what's more, the answerer loses out on the knowledge they might well have gained from other people's answers to the same question);

it is unfair - they are taking your time away that you might otherwise spend on the community;

answering private questions only encourages people to ask more one-to-one questions when actually they would be much better off throwing their questions open to whoever feels able to answer (they have no way of knowing whether you're available to answer or even whether you're the best person to answer);

it puts individual users under psychological pressure to fulfil some kind of imagined duty to their unsolicited correspondents and this leads to stress and even total withdrawal from checking email;

if people are prepared to do things free on private request that the questioner is not prepared to publish, that may be diverting potential income from professional freelancers, and that means these people are exploiting your willingness to help for their own economic benefit - there is a big difference between volunteering for the community and allowing oneself to be exploited.

For all these reasons and more, I have said no to private questions (this is on my profile). Some people think that's mean and selfish of me, but I guess that's their problem.

Turning to why they do it ...

Most of them have no conception of the time pressure that the person they have written to is under. They think it doesn't hurt to ask a "quick favour", but they just can't understand your situation.

Some of them really are just exploiters - they see a chance of getting professional, private service without paying for it.

Sounds like these people have good knowledge that you could be a frequent user of yahoo answers and probably very accuret and helpful with your norm answers on here.

Seeing as to where so many people dont have :"interesting" enough questions to have people click it and answer it they might search a main personal opinion aka you :)

You should feel honored lol

I think that it is a compliment to you. They want YOUR answer. If they post the question, then maybe you won't spot it and maybe you won't be disposed to answer it. A lot of people who write to me want to discuss things in depth and I find myself delving into arcane areas of knowledge and re-emerging quite stimulated by the experience. I decline to respond to mail if I feel that I am simply being used by an ungrateful student who is too lazy to do course work, or if the person is being frankly abusive. If the person concerned is simply by-passing the Y/A system, I invite them to post their question so that perhaps they can have a range of answers, particularly where it is a question which is open to discussion and debate. I must admit, though, that I answer far more of these questions than I ignore.

They see you as a good resource. You are very knowledgeable so they probably want to ask you somethings that they want YOU to answer, since they don't have a dependable way to know if your online or not. Some users do this to conserve some points.

I haven't had that happen. Once in a while someone emails me with a question regarding my answer to their question... but not directly asking their questions to me.