Modern Life is so complex. And in many situations , telling whether they're zero sum, is uncertain, and hard. Look at the debates here for example. So people default to what they already generally know. They know that society is very competitive. The job market is very competitive. The mating market is very competitive. Zero sum games are everywhere. So it's no wonder they have this cognitive bias.
On an anecdotal level, while thinking that everything is zero sum all the time probably isn't good, I find that the majority of the people I interact find it very difficult to imagine that anything is "zero sum." Since they associate "zero sum" with mean things like harsh competition. It seems like that's its own sort of "bias."