The problem is that this model loses most of the benefits of the Firm (economies of scale of labor).
Let's say you have some money and want to make something happen. You find a developer and an infrastructure person. Now you have to negotiate a price with each of them for some defined output (and unless you're technical you probably need to first negotiate with the dev to get her to produce an infrastructure plan, then find the infra person and negotiate with him, and then negotiate with the dev to get the work done).
Ok, you've spent a bunch of time negotiating individually and defining exact output specs that withstand contractual scrutiny. Then you get partway through development and learn something new about your market. You decide that you need to change your product a bit, so now pause development and negotiate a change order with your dev. You come to an agreement but the change requires infra changes. So you go negotiate with the infra person but you can't come to an agreement on the changes required.
Now you're paying the infra person to finish something you don't want any more, the dev can't do the additional work she's willing to do because the infra won't be there, and you're stuck building something that you don't even need. This doesn't even take into account that this model forces every single person to be both a negotiator and marketer of their own skills, in addition to whatever technical skills they have (comparative advantage shows that they should each just do what they do best). The obvious next step is for groups of people with complimentary skills to get together and negotiate projects together and from there it's a very clear path to ending up with salaried workers again.
The advantages of the Firm are just so great that even with a ton of waste it's still better than everyone individually negotiating their output. Look at the consulting companies for an example, even though they negotiate at the project level they still pay people salary. They know exactly how much bench time each person has and they still use that model over hiring for each project based on the project's output.