Simply put: Software companies aren't all the same. Some sell to consumers (B2C), others to businesses (B2B), and others sell to both.
You won't find many B2C companies with sales people, due to the cost of sale outweighing the return from one customer. Oppositely you won't find many B2B companies without them, as cost of sale is built into the price (read up on high vs low involvement products if you need to know more). B2C's typically rely on marketers instead, as they can help spread a message across a broader prospect base, keeping the cost of sale down.
Sales is a set of activities a person does to compel someone else to sign a contract. So all those B2B startups probably do have sales people (eg people selling), just that they may be the founders or developers helping prospects with the hope of them converting to customers.
Finally, the reason salespeople aren't running software companies? Depends what you mean - no they may not be founders. But once a company is growing, and trying to grow faster - show me which company doesn't seek advice from their sales and/or marketers on how this should be achieved.