Because it says if you want those gender roles you must be that sex.
You can't be a man who likes floral material, crying at films and cuddles kittens. That must make you a woman.
You can't be a woman who is competitive, ambitious, opinionated and a leader. That must make you a man.
Some trans people may say this. But is it intrinsic to trans.
Those with body dysmorphia might think this sort of thing is not "core". They are part of transgender after all.
Even for the rest, there is still some assumption and maybe some brush-tarring here.
Here is a possibility. Someone has an inner sense of being a man or a woman. This is not as a result of weighing up which stereotypes they prefer, or because someone told them they must be this that or the other due to their preferences for things stereotypically associated with one sex. It is just there within them.
They might then adopt some of the conventions of men or women in the society they live in. Just like many many "cis" folk do. But not always, see Danielle M. As I say, if you object to the stereotype that women wear dresses then by far the biggest perpetrators of it are "cis" women.
You can assume that trans are only trans due to a preference towards certain stereotypes, but that is an assumption. You can then conclude that transgenderism, at it's very core, therefore "enforces" gender roles, but it is not exactly a done deal that this is true. Yet it is assumed and all trans people get tarred with it.