YC doesn't have some sort of moral objection to solo founders. They simply exist to find and fund huge companies, and smart risk/upside management is fundamental to winning that game.

Being a solo founder represents added execution risk, particularly in the early stages of a startup -- which coincidentally, is precisely where YC funds most companies. They aren't the only party with this bias either. Downstream investors share these concerns, and when you add financing risk to execution risk, you start to get the picture.

Of course, all of this can be overcome as a solo founder, and YC does fund solo founders at a roughly 1/10 ratio to co-founded teams. However, it usually means de-risking yourself in some other area (i.e show tons of traction, have a proven track record, already have an exit, etc).

It's not a crime to be a solo founder, and plenty of great companies get built that way. Just understand that without the above, you may not have a venture-fundable company.