There are 3 reasons why investors prefer 2 founders (although it is not strictly "necessary"):
1) With one founder, if something happens in that person's life the company may crash.
2) Two founders probably have a more complete skill set than one (either the sales vs ops or biz vs tech). Partners can share the load and help each other down the road.
3) If the one founder has not been able to recruit a co-founder, that might mean the founder does not "play well with others" and wants to be a dictator, or it might mean the idea has a fatal flaw that repels knowledgeable cofounders.
So, no single rule or characteristic is ever "mandatory", but investors often feel more comfortable with companies that have a more complete team, including a duopoly at the top.
I always remember a fortune cookie I got once: "No success if you work alone. Take a partner."