Why investor require start-up having a co-founder?

I contacted a lot of investors and accelerators. I found most of them don`t care about the product itself too much, but strongly requiring start-up should have a co-founder.

Why?

Perhaps some of founder them don`t know about programming or designing, so they may need a co-founder like CTO to responsible for that.

But what if the founder is good for both programming and designing skills and more, managing the start-up petty by one person?

I think finding or hiring an excellent A-player co-founder and employee is hard and impossible for start-up in early stage, but it may be possible after the start-up received funds from investor.