Because some people don't use that term to describe themselves either, even if that is what they biologically are. CIS is a pretty good term, because it's very technically correct. Sadly many have been on the receiving end of debates where it is used a a slur, and considers it hurtful.
Personally I mostly see it used in anger, so I wouldn't use it, unless doing a biology report.
It also has the weird problem that you mostly see it in a "do you identify as cis", which I at least don't... I am cis, but that's not question. This leads to some really weird situation where you have teams of larger middle age straight, and cis, men, but you have one or two that answer that they don't identify as such.
This is very much off topic, but I'd personally I'd prefer that we just stop labeling people. Just use their name, it's fine, you don't need to know or even understand the gender of the person who makes a repairable flatpack toaster. It's an cool project regardless of who made it.
Because some people don't use that term to describe themselves either, even if that is what they biologically are. CIS is a pretty good term, because it's very technically correct. Sadly many have been on the receiving end of debates where it is used a a slur, and considers it hurtful.