Right, but that is just one way to do it. In the hypothetical scenario that every investor chased only $10B+ potential companies exclusively, $100M-$1B companies would be super cheap and those could be profitable wins.
But if you're going after a niche market -- a computer vision-based tennis coaching app, for example -- your valuation will necessarily be lower than a company with broad appeal.
So instead of saying, "No, that's a small market" you could invest at a lower valuation and still make the same or higher return as with a unicorn.