This is caused by the fact that the online markets for freelance coding are open to anyone. Since anyone can post, naturally you will see a lot of impossible jobs.

Since anyone can bid, naturally you'll see bids on these impossible jobs, too.

But, underneath this superficial layer of incompetence lies a good selection of developers and clients in each of the freelance sites. Having done some work on more than one of them, I can say that as a developer, the key to finding actual work is to take the tests, have a portfolio, and wait for clients to contact you. If you must bid on the jobs by searching, search hard for the realistic ones and bid appropriately. The clients that understand their work will not usually go for the lowest bidder just because. They'll go for the one they have the most confidence in.