House Owners that prefer single travelers vs couples. Why?

I have seen this many times while I'm applying. I apply as a couple and think it's a better option for the HO. You get 4 pairs of hands and if someone gets sick or needs to run an errand you have the other.

I get some homes only have a single bed or are a studio and just don't have the space. I've also seen female HO prefer a single female sitter. I get that one too.

Other then those specific scenarios, why prefer a single sitter instead of a couple?