Because crossdressers REMAIN cis, and that the act of crossdressing is merely one of performance. They can switch off being a woman or a man. Trans people do not have this privilege. Trans people can't just wave away being trans, or rather, society itself makes it very difficult, as it constantly puts restraints upon their lives. This is, once more, about whether there is a trans essence, a checklist that trans people can check off and then they're Trans and they're accepted. And again, I am telling you: there is no such thing! And even if there is, trans people under no circumstance should let cis people decide that, because then their acceptance will be inevitably bound to them (even further than it already is...). So, if a trans person tells you they're trans, you better believe them, otherwise, you're just showing grave disres[ect.