You could not have said it any better. I've spent the last year learning this skill and it is a lot harder than one would generally think. It isn't something that you can just do as an afterthought after you are done hacking for the day. There is no reason why someone who is really good at it would work for anyone else.
There are plenty of so called "internet marketing gurus" (the Frank Kern's and Mike Filsaime's) who teach it but most of them are no good. They have only ever sold internet marketing informational products to other aspiring internet marketers. If a potential hire has only sold internet marketing products this is an immediate 'no hire'.
"There is no reason why someone who is really good at it would work for anyone else."
That's really just a false statement, in my opinion. That's like stating why would any good programmer ever work for anyone else?
Honestly, there are a ton of reasons.
For example, a programmer may wish to work at Facebook so their code is put in front of 500 million users. If they hack something on their own, that's likely not going to be the case.
If a marketer wants to test his/her conversion theories against a large volume of traffic, they either need to spend LARGE amounts of their own money for paid traffic, or build up a large audience of their own. Again, an unlikely scenario.
Whether you're a marketer, or a programmer, or whatever else, working at a company/startup can provide you with opportunities that do not exist on your own.
Those are some of the prime reasons people work for others, no matter their trade...
And there is the distinction. Spending large amounts of their own money to get massive traffic is EXACTLY what good marketers do. They can get the exposure a company like Facebook gives developers. A great marketer can build something in a week that can make him hundreds of thousands of dollars. The best hacker, unless he has marketing support can not. This is not to say hackers are less valuable than marketers, nothing is further from the truth. But, in some cases, top marketers can create a lot more value for themselves more quickly than top hackers.
I would say that hackers are a lot less valuable than marketers. Of course at the top levels you're measuring apples and oranges but if you need someone to create a product and you're willing to pay $150/hr you can probably hire a good hacker. If you need someone to create a marketing strategy you may have to pay $500/hr.
There are plenty of so called "internet marketing gurus" (the Frank Kern's and Mike Filsaime's) who teach it but most of them are no good. They have only ever sold internet marketing informational products to other aspiring internet marketers. If a potential hire has only sold internet marketing products this is an immediate 'no hire'.
Good luck finding this person!