Why do people continue to post questions on here that are obviously for their school/college/uni work?

Because they want you to do their homework! I wish we had Answers when i was at school!

because they are hoping someone will do the work for them why else?

Laziness!

For the simple reason that if you never inquire you will never find out. Sometimes the books can be very vague. I say good luck to them.

why not im not at school now but if i was id try it

I'm not sure I see it as much with the college & university kids... however, the kids in the lower grades (high school / middle school) are certainly taking advantage of this in the Math, History, and the Physics sections of Y!A They do it because people continue to keep on giving them the answers & doing the work for them. (I've been guilty of that a couple of time). In some cases - yes, they're ligitimately stuck and are looking for some help to complete an assignment or some homework... but more often than not - it just looks like they're asking other people to do their work for them. (In which case - they'll end up failing those type of questions when they show up on exams... so I guess that all works out in the end, eh?) These are the same folk who could just as easily find answers if they just typed in their questions on a "google"-search engine or something like that... or just looked it up on Wikipedia... but no, they want to be spoon-fed the answers and some of us keep on giving into this & answering them instead of making them do it themselves.

for help when its too difficult

Because they are too lazy to read a book or even use a search engine. It's all about the easy way out.

Because their attempts at research have failed, or because they are too lazy to do it for themselves. But, mostly because Yahoo Answers is all about asking questions. ANY questions.