Not being rude: Why are most autistic people so horrible at getting social cues?

I mean why? They just don’t get it! When someone tries to explain shit and they always overthink and aren’t really direct. They don’t LISTEN.
I have an autistic cousin and she is bad at social cues. Not saying it to be mean but it’s the truth. It’s so hard to talk to her so I don’t even bother and I just ignore her. I’m not obligated to talk to her and hang out with her.
One time my brother asked if she wanted to go to a party but she said, “I can’t go. It is too old and I don’t wanna go outside and it will be too loud, craziness and I don’t like too many people and I might go.” She kept running her mouth and said something rude and upsetting to my brother. We just looked at her annoyingly. He sternly said, “Forget it! Why waste my time asking you! Rude!”
I scolded her big time. I said, “You could have just said, “No thanks” of “I’m not up for it.” Short and direct. You didn’t have to be so rude and run your mouth.”
Sorry but if autistic people can’t get social cues or have a proper conversation, I don’t wanna talk to them! Just being honest!