A full partnership from the start makes sense...from the start. But we've spent two years developing the idea, doing market research, hundreds of interviews, pivoting, building an MVP, recruiting the first dozens of users, etc. We know many things we didn't at the beginning, like what it is that users want, how to build it (where to get the pieces,) what order to build it in, where to bring the users from, how to make money from it. That is what we have.
Obviously, without a CTO/developer, this thing is not going to move forward. But on the other hand, we are not in the idea stage, sitting around waving our hands and going "wouldn't it be cool if..."
Since there is money on the table now, it would not make any sense for us to say, "ok, you're a developer, please join us as CTO, sight unseen." I've seen lots of startups end up in a bad place with this approach Some sort of limited project to see if we're right for each other, with the developer getting paid something for his time, makes more sense.