The startup script is actually quite simple. At the initial stages, a startup is nothing more than a few individuals coming together working on an MVP. Then later, that MVP gets some traction, which proves customer demand. That demand is rewarded with cash or investor funding, which fuels growth. That MVP then becomes a company where it now can sustain itself, its employees, and founders. That company then gets more revenue, which either fuels growth or investor capital is injected.
The most critical moment in that early life cycle is where the MVP gets market demand. If it gets demand, then the rest of the script follows. If not, the founders should find another MVP where demand is higher.
The MVP gets market demand because the founders can get users or consumers to use it. But oftentimes the MVP is so raw where there is just a back-end and a barely-looking front-end. Most anyone with some aesthetic sense would not be able to play around with the front-end. How does the MVP gain market demand then?
At that critical juncture, the market demand is created via bruteforce. The founder has to understand his market uniquely, his customers, and get his customers to buy or use his product. Tactically this means convincing them that they have to use this. There is no other way.
The founder cannot outsource demand creation at this point. The MVP is so raw that no one can market this effectively except the founder, who knows the product inside-out and has the passion to convince naysayers to believers.
This is precisely the reason why the initial market has to be marketed to the founders community. Only the founder can convince like-minded people in his circle to use the MVP. Only his community would give the MVP a chance to succeed.