"So why do we see so many tech startups with only dev skills and no selling experience?"
My own anecdatum is that I see a lot of startups with selling experience but no dev skills. They put up a landing page that spams you into signing up for their MailChimp newsletter, then if they ever get around to actually releasing a product, it's a piece of garbage.
I think, if you're sitting in incubators or coworking spaces, you've specifically selected yourself into an environment that appeals to developers, and your observation will naturally follow. Coworking spaces are places of doing. In my case, my selection bias is because I read Hacker News and Reddit. Link aggregator sites are places of telling. Of course that is what we see.
So why are so many teams inadequate in some area, whether it is sales or development or design or legal or marketing or operations? Because everyone only knows what they know how to do, and we've all been told, "if you build it, they will come." What is "it"? We're never told. No, scratch that, we're told it's whatever your "passions" are that you're supposed to "follow". Our popular media reinforces the idea that all that is necessary is perseverance inside ones own sphere of influence.
Most people, regardless of background, aren't very aware of the role the advertising plays in their decision-making. We all like to think that we make our decisions based on merit. A lot of people have business ideas that don't go anywhere. They might even write massive business plans and talk to their friends about it, because they heard somewhere that that is how you start a business. For a developer, however, we can actually *do* something about our ideas. So take the normal naivete towards success of the masses and add the ability to actually build and you end up with a lot of 1-star projects on Github.
There is a joke in Computer Science that somewhere in Russia there is a kid who wrote a program that proved the P=NP conjecture on his Cold-War era PC clone, and just never knew what he did to tell anyone about it.
So few people know what it takes outside of their own skill set to be successful in business. It's a whole Venn diagram of skills that have to intersect to find that way. If you're in one bubble, you might only see a few of the other bubbles and not the whole set.
That said, what is that set? I think there are certain *jobs* that need to be done, no matter what, but who will do that job is really going to be up to the specific people you have. I know developers who are excellent salesmen, have a good eye for design, and have the hustle to spam the tech press enough to get on top of the ever growing startup pile. In the early stages, they aren't going to need *anyone*. There are three major tasks of sales:
1) get new people in the door. This is obviously marketing.
2) get them to buy. This is obviously sales.
3) and get them to come back for more. I think this is doing a good job of whatever you're selling. In tech startups, that's development. It'd be different if you were in a service industry. Regardless, you get people to come back for more by giving them more than what they expected.
If you're doing a startup, I think you have to do all the jobs yourself at some point, or else you won't know how to hire for it. You won't know what all the jobs are, even. Or else you end up in the situation you've described, either without anyone running sales, or nobody doing good job of it. It's when the whole thing grows bigger than you, that's when you need to find people. And you'll know who you need to find. You'll know, "I suck at making things people understand, maybe I should hire a copy-writer." Or you'll see, "I'm exposing myself to a lot of risk with all this user data, maybe I should have a lawyer review my TOS".
The only time I would consider partnering with someone is if we both know each other well and we both are confident that the other is particularly skilled within their wheelhouse, and that our skills complement each other. But Paul Graham told everyone you *have* to have a cofounder. He meant that you need to build to the point where you have that person you can trust. But a lot of people are putting the cart before the horse on this issue. They have no clue what the work takes, so they just stick a name in the slot. Of course they fail.