That's because transphobic snowflakes react to it. They want to ignore the humanity and such of trans people. So they refuse the identification of cisgender. Thus they get called out.
Getting called cisgender, asking what it means, shrugging and going "ok".
That is the reasonable reaction.
Same as getting called straight/heterosexual and then just going "ok" is a reasonable one.
But transphobes hate getting termed as cis, and then out themselves.