Is there a need for "outsourcing founders team" thing?
Also, investors have things thing about inventing the future by funding it. Generally, individually or as a group, they'll decide something is going to be the next big thing and start cultivating deals in that area. One you can see happening in real time is space: Randomly in the last couple of years, investors have put together huge funds around space ventures, even though serious commercialization still seems to be a ways off. Even OP referenced it in his blog post.
As for why a founder vs a compensated product owner? Because a founder presumably has spent a lot of time thinking about the problem and is intrinsically motivated to solve it--so much so that they've done all the legwork to assemble a team and gather enough data to be investment-worthy. If as an investor you're wrong about the opportunity in this space, it's a whole lot of risk and cost and time you don't have to take on yourself. By advertising a pot of money, instead of searching for the right person, the hope is that the right person and team will eventually come to your door.
In my experience, you do not find such a person for hire.
To an outsider startup ideas usually look like barking up the wrong tree.
Good luck finding a talented, motivated CEO willing to enthusiastically bark up the wrong tree for years.
Again, isn't it easier to find an experienced person who likes your idea, than find a person who both has the same idea and experienced and motivated enough?