Construction projects, if not managed by someone who knows what they are doing, absolutely do go over budget and overtime. The difference is that on construction projects, construction project managers who work for the company commissioning the building work are experienced project managers who have normally been working in the construction industry and are knowledgable and skilled in many areas of construction and do know what can be delivered, what should be delivered, and what they need to go when they encounter problems and project slippage.
In fact, in construction there is actually MORE outsourcing than in software projects, because in actual fact you'll contact out specialise work (e.g. Electrical work) who then often five work to subcontractors.
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.
In fact, in construction there is actually MORE outsourcing than in software projects, because in actual fact you'll contact out specialise work (e.g. Electrical work) who then often five work to subcontractors.