Are there perhaps political reasons for it? Do investors prefer having two or more founders they can divide if necessary? Does it increase "bus factor" or otherwise derisk the venture?

My opinion: two or more founders who are really invested and work well together > single founder > forced cofounder relationship. I say this as a single founder.

The hardest thing about being a single founder is cognitive load. I don't necessarily mean time. There are usually enough hours, but switching between at least five hats really strains the brain. It can also be hard to maintain energy. I find groups with other founders and working in a good startup oriented coworking space very helpful.