Where I live it is very ususal to just hire an architect and then a general contractor. So the party that commissions the project (has the "idea") really has little knowledge at the start of the project. But of course they depend on their partners to handle all the project management.
Which all is just to say: I think it is very reasonable and common to only bring the idea and then hire someone to do the work. The problem with what is mostly called "outsourcing" in the software industry is not the fact that you want to hire someone, but more in how careful you select your partners.
If the secret sauce of unicorns were mere commodity shovelware, I would agree. There's nothing wrong with outsourcing non-core competency side jobs or commodity shovel work, happens all the time.
Or rephrased, is a tech unicorn not made out of tech?
I believe it is not. There is a user behavior and a data flow that go together to get traction. Marketing is probably the most important part. Luck being the 2nd. Tech may be way down there, alongside carbon paper.
Which all is just to say: I think it is very reasonable and common to only bring the idea and then hire someone to do the work. The problem with what is mostly called "outsourcing" in the software industry is not the fact that you want to hire someone, but more in how careful you select your partners.