I believe it's because PG is opposed to the idea and HN has inherited that philosophy.
It's worth pointing out that Thomas Edison, Edwin Land (Polaroid), George Eastman (Kodak), Nicola Tesla (lots!), and Walt Disney were all single founders.
Completely different datasets here. The research was self-reported data from surveys sent to Kickstarter "founders" across all categories. Y Combinator uses actual portfolio data for tech-based startups looking to create large-scale, $100M+ businesses. Success in the former is largely not going to predict success in the latter.
It's worth pointing out that Thomas Edison, Edwin Land (Polaroid), George Eastman (Kodak), Nicola Tesla (lots!), and Walt Disney were all single founders.