Why is "estar" not used here in a description?
This is describing a permanent quality of the person, like being tall, dark, and handsome.
I'm a native spanish speaker and I cant say to you that replacing "eres" for "estas" is completely right and there should be no reason why it would be wrong apart from it being a more informal way to say it.
Changing ser to estar isn't about how formal it sounds, but how temporary or permanent. In some cases, it just doesn't make sense in Spanish.