It's not that we always need to use cisgender. It exists for when it's important to distinguish. You don't go around referring to people as heterosexuals, but the term exists when it's relevant. You don't refer to milk as "whole milk" in normal conversation, but the distinction is used as necessary.
I see what you're saying and understand, but it's not like there's a default milk that dominates 98% of the market lol. In instances where it would matter, you've probably already identified the variant. But no, I get it. Maybe not in a typical social interaction, but for the sake of scientific study or comparison, I see why it is used. But it's only practically relevant when comparing beyond biological gender and sexual orientation.
Perhaps, but considering this topic is expressly about transwomen competing against ciswomen, this would definitely be one of the times where its use is appropriate.