That could be the case but my theory is that it's about control. Investors funded a solo founder before and then when they said to take some action (sell/pivot

That could be the case but my theory is that it's about control. Investors funded a solo founder before and then when they said to take some action (sell/pivot/scale up) the solo founder didn't do it. They said damn this guy owns 70% of the company after Series A, we can't make him do anything we want! Now if there were 2 founders each founder would only own 35% of the company, and if they could convince one of those two founders they could get what they want. All investors want cofounders because they want more leverage. So if some investor is urging you to team up with a cofounder to 'complement your skillset' it might actually be that they aren't confident in your leadership and want an option to override you.