The short answer is that the statistics is clear - outsourcing significantly decreases the chances for success as compared to a dedicated team.
Why this is so is not entirely understood, but the most oft cited reason is simply that freelancers and even full-time employees with high salaries can never be motivated enough to see the project succeed, because they are being paid by the hour, or for meeting predefined milestones, or for some other meaningless (for a startup) metric.
Why is motivation so important? Because creating a new product for an unknown market requires creative thinking, deep understanding of the task and the goals, working unusual hours, and many other difficult tasks that no salary can induce or compensate for. A freelancer will only do the minimum of what you ask of him, which won't be enough because you probably don't know yourself what you need. On top of that, a freelancer will quit as soon as he finds a better deal elsewhere, leaving you out to dry. Don't expect the next freelancer continue from the same point as if nothing happened - no, you'll probably have to start all over again.
Company performance bonuses and stock option plans were designed specifically to solve the motivation problems, but they still pale in comparison to the hefty equity share.
In any case, outsourcing is expensive. I've seen people paying tens of thousands of dollars just to get a prototype (that no one wanted to use), whereas a technical co-founder would do the work "for free". Same is true for a bizdev co-founder. Sure, if the product becomes a success then a co-founder becomes more expensive, but given that most products fail, if you count expectancy (especially accounting for the decreased success chances with outsourcing) then a co-founder is much cheaper on average. And investors have done the math.
And finally, working on a project alone can be unbelievably lonely, and no one can alleviate the loneliness as well as a co-founder can. Neither employees, not investors, nor clients, no one. One reason that one-person teams fail more often (outsourcing or not) is because the founder has no strength left to continue, and no moral support to get that strength from.