Wow, passions running high in this thread. 
Hey, pro illustrator here. Did a stint of about ten years in the business and now I work part time as a creative consultant because full time freelancing as an illustrator gets really exhausting and has no career progression. But anyway, I've done work for the BBC, Nintendo, Screwfix, English Heritage, a bunch of councils and tech events... Basically I've been in the biz.
As far as I have learned from my experience, it's very rare for a successful illustrator to be just good at drawing with zero marketing acumen. It's frustratingly more common to see somebody who is a bit of a hack and not particularly great but really good at marketing themselves... but saying this is good marketing simplifies the issue too much and is really not giving the less skilled "hack" artist credit for how hard they're working.
Being just really good at art isn't necessarily helpful to a person who wants to buy your services for a specific purpose. From the client's perspective, they want:
- Somebody who is proven to get jobs done and not vanish on people.
- Somebody who is easy to find because their art is put in places where non-artists looking for art will find it.
- Somebody who is easy to communicate with and makes them, as a non-artist, feel like their vision is being realised and that they're in confident hands but are being respected, not talked down to.
- Somebody who makes decent enough art in a style that's appropriate to the job.
- Somebody who is available to start the job when they need the job doing.
- Somebody whose prices fit the budget.
So, given all these factors, you can easily see how a person who actually isn't a particularly amazing artist, but rather just draws say, a very simple cartoon style, but who is easy to find because they have a professional looking website with a simple URL, advertise themselves widely on social media and have an active agent, has worked hard to get a reputation for finishing jobs, charges standard industry rates with neat invoices submitted on time, and is very pleasant to talk to and makes the client feel good could absolutely become very professionally successful!
In fact, I would much rather work with the person described above than somebody who is absolutely amazing as an artist, but continually misses deadlines or ghosts on projects, is pretentious and talks down to people, won't take feedback, instead trying to boss the client around about why their creative decisions are right, and isn't open about their charges. And an artist who has no social media presence, no website but like... an outdated artstation profile somewhere... well how am I supposed to even find that person! More than once, I've tried to recommend a friend to a client (because if a friend is clearly way better suited to a job I'm unsuitable for, or I'm unavailable, I always recommend people) only to discover their online presence is an embarrassing shambles and I have to be like "Oh er... okay I know the site looks a bit sketchy, but really, they're really good and professional! I swear!"
When I say that a person can get by being just okay at art. They have to actually still have a polished finish. I've actually seen quite technically capable young or new artists lamenting why a person with inferior anatomical drawing, compositions and perspective etc. is getting work and they aren't. It's usually polish, and it's usually something about their line quality and the inconsistent level of detail throughout the piece makes their pieces look unfinished, or their values are poorly balanced, or the characters are all standing face-on in a really flat looking way.
What I'm saying is... don't underestimate hacks. Hacks may not be the best at drawing, they may not be the most creative with their ideas or their layouts, but they work hard, makes themselves easy to find and understand, and they get the job done, consistently, to a professional level of polish. I wouldn't be surprised at all if there are people who consider me a hack! 