For most consumer products, it's less about domain expertise and more about capturing "lighting in a bottle." The founders of YouTube weren't necessarily video experts and the founders of Instragram weren't necessarily photos experts, but when their products started to grow quickly it was clear they had done something right.

For areas like healthcare, nuclear energy, artificial intelligence, biotech, and many areas of the enterprise, being a domain expert (or acquiring domain expertise) is essential. Often, it requires money to even get the product to the point where it can be launched, and so the only way to establish credibility is through the team.

In any case, you don't have to be an expert in entrepreneurship--you can generally learn business as you go along.