I know NSTB as a very appreciative client (who liked my work but not my prices) and Meo as a legitimate writer (whose work I don't believe I've ever seen firsthand). In my opinion, there are enough
real dirtbags who post here that you two shouldn't fight one another.

It's just very impractical for a writer to do the work before payment for the same reasons that the vast majority of most types of consumer transactions and services conducted online are always paid up front. A writer doesn't actually have any practical resource to recover unpaid bills because: (1) Many times, clients prefer not to even disclose their identities; (2) Most of our clients live way too far away (including overseas) to pursue a $100 or $500 or $1,000 claim by travelling to that jurisdiction to go to court over it; and (3) Even if they're local, we'd still lose much more money in time and expenses pursuing it than just letting it go.
It's also easier for a client to guard against getting ripped off than it is for a writer. A client can (and always should) test any new writer with a very short paper before trusting the writer with a major pre-paid project. Once you get a good piece of work, you can be fairly certain that you're dealing with a legitimate writer who actually does this for a living and isn't in the business of ripping anybody off. It's extremely unlikely that a writer is going to provide good work on a short project just to bait you into prepaying for a much longer project with the intention of scamming you on it. Writers also have a reputation to protect and a strong incentive to satisfy their clients; conversely, clients typically don't have a reputation to protect or any vested interest in maintaining the same online identity for many years.
The reverse is much more of a risk. Specifically, the only two times I was ever stiffed by a client was by someone for whom I'd written several dozen projects (maybe more) over several years. Once in a while, the person would ask to pay after delivery and I didn't have a problem with it, reasoning (precisely) that it was a long-term client who'd always paid me up front and who also paid up the few times I'd previously accepted payment after delivery. The person actually asked to pay for the last
two projects afterwards. It turned out that they were the very last two projects of her degree and instead of paying up on the date promised, she apologized for the delay and promised payment "next week." After next week came and went, she wrote me a long email apologizing again, profusely, and explaining how tight her family was financially and that she'd "try" to make good on the debt as soon as possible. No payment ever came. About a year or so later, the same person had the nerve to email asking for a job application letter (about
integrity for a law-enforcement-related job, no less), promising to pay up front for it but without any acknowledgment of the unpaid debt. That type of thing happened to me another time with a different client for whom I'd also provided many projects for several years.
You also have to keep in mind that some clients who are totally new to this whole process don't understand (until it's explained when the issue first comes up) that requests for some types changes after an essay has already been furnished are totally unreasonable, such as where the client tells you only after the fact to focus on some specific area or element that was never part of the original order specs at all. When that happens on a pre-paid project, we just explain politely that rewrites for our mistakes or omissions are always free, but that changes and edits the client only thinks of after the work provided perfectly meets the original specs always have to be paid as additional work. Human nature being what it is, some clients would unfairly demand the additional work and refuse to pay for the project until the additional work they have no right to demand fairly is provided.
Some writers who really need the work more than very established writers may still knowingly choose to take the risk of not getting paid once in a while in their effort to compete for business, and that's their choice. But those of us who have been doing this for many years just don't need to take that kind of risk. I no longer even allow established clients to pay after the fact, because of the specific type of experience that I detailed above. That doesn't mean a client is "wrong" or nonsensical to
prefer paying after delivery; but you have to understand why that's a condition that really narrows down your pool of potential writers and that you're excluding many of the best and most experienced writers who are in the greatest demand because they're good at what they do and that you're increasing your chances of getting a writer who is much less experienced and in less demand than those of us who don't need the work badly enough to take any risk of not getting paid.
None of the above is intended as an insult to or criticism of any legitimate writer who chooses to take that risk, and there may very well be a less-competitive niche to fill by offering delivery before payment; but that niche has a lot less competition from experienced writers for some very good reasons.