I don't have citations for you but I don't think it's anything like a "personal preference" - YC has tons of data on this. But it's fine to apply as a solo founder—many do get funded; and you're quite right—the risk of picking a wrong cofounder is also high and is not something one should do lightly (e.g. just to try to improve a YC application).
The thing is, YC heavily signals against solo founders.
So it seems like there could be a lot of adverse selection in your data where good solo founders don’t bother applying, or good solo founders take on value destructive cofounders just to satisfy YC.
That’s then a vicious cycle where YC sees increasingly lower quality solo founders which reinforces their opinions and data about solo founder outcomes.
