worth considering provided they charge considerably less
This can be one reason. But it's not the only reason to hire a freelance writer.
worth considering provided they are legitimate / reputable.
Major has answered his own point. The entire issue is with the companies being 'legit and reputable' vs. 'the ones that misrepresent facts'. We know very well that by the hour more and more scam companies are being established on the Internet. It leaves the legit ones under extreme pressure to compete.
Above all, the most challenging part is 'how can a client reach a 100% legit company?' Google? No way! Lethal SEO methods are perhaps giving an edge to those companies that are probably 'not legit'. There is one famous company that flaunts on its website a case it won against a misrepresenting company. My question is what guarantees are there that legit companies (only a very few in number) attract more global clients (to earn more profits) than those that are not 'legit'? No one probably even knows how many of the two kinds exist over the WWW.
This leaves us with more of less the same ambiguity that the clients may confront while dealing with a writer or a company for the first time.
Legitimate companies have some sort of a hiring process.
Which legit companies? I do not want to know. It is the clients that must understand that 'these legit companies' have 'some sort of a hiring process'. Unfortunately, the legit ones seem not to be doing a great job in assuring the global clients of their legitimacy.
This forum has not provided the support that was perhaps envisioned while it was being developed. It ended up becoming a marketplace in itself. No one should forget the legal actions, revenge, scathe, and bloodshed that took place in the recent history here and in its neighborhood.
More and more posts about company reviews (negative and positive) leave the global customers in more gray.
Dr. Paul Steward.
This is a valid point. A disposable email never guarantees who's actually who. What about the claims on a company's website (legit or not) "We employe PhDs to handle your order...., and so on"?
In both the cases, it is the client's ICTs skills that must be put to work. If so, then legit or illegitimate companies, scam or honest writers sail the same boat.
who don't misrepresent their skills, location, or credentials.
True. Legit companies don't 'probably' do so. They should go out and tell their potential clients about it. Can they do so? Yes they can. But it seems the pendulum is swinging the other way more.
With their physical address in place, along with other legit details, they ARE NOT 100% transparent about (i) the qualification of their writers and (ii) their quality standards.

A scam freelance writer is a tiny bit as scamming as a BIG company. The former may take away a small chunk; while the latter can cause the client a huge earthquake.
The problem seems to be getting worse by the day, isn't it?
My personal experience as an ESL freelance writer.
It has always been my professional curiosity to learn, when a client approaches me for some work, that why he/she is willing to hire me rather than go to a legit company.
I would list some of the reasons:
1) Communication barriers, delays, and semantic noise. "They don't really understand what I am trying to say".
2) Paying the whole amount upfront. I don't know about other freelance writers, but to me getting paid in installments is not an issue.
I remember a client. He was well aware of which companies were legit and which were not. He had a PhD thesis to go. I asked him the same question. He told me that to hire a company he would have to sell his apartment or he would never be able to pay them in one go.
The amount he paid me for per page compensation was almost similar to what that company would have charged him. I won the project because I was willing to work in bits and pieces that assured him an effective monitoring of my work as well. I beat the giant here!
As my clients have my necessary contact details with them, I am there to assist them in sheer ABSENCE of a company rep. relaying messages back and forth. They find it quite satisfactory to discuss 'matters' directly with their writer - in OUR CASE.
3) Too many of these companies. Don't know where to go.
Too many of the writers may also be a case here. But there is a difference. There are writers with virtual connections and reputation, and maybe relatively better skilled (ESL/Native) vs. those that lack in this area.
The clients are living human beings. They may employ some effective strategies to pin down a legit freelance writer (I won't discuss what strategies - or you would think I am advertising my services here). It is quite obvious to understand that a sensible client who faces the issues mentioned above can conveniently tell after initial communication if it is THE writer with the SKILLS that he/she wants for their work.
The same is probably not possible with a company since all they can tell them is 'they are legit'.
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That said. Major has time and again shown his contempt to the "disposable email".
With an honest freelance writer, this 'disposable email' is their real strength. It is their trade mark. To them it is no more disposable. It may be disposable with the scamming writers. So the same goes for a domain name when it comes to a scam company.
Thus, it would be appropriate to suggest that these are two different niche markets with specific strengths and weaknesses.