Why do some of us forget basic Islamic manners?

As-salamu alaykum. I wanted to share something that hurt me today and ask for thoughts. I was at a café with family. My mom and aunts arrived first and she had already bought a cake. Since I was with my cousins I offered each of them a bite, which felt like the polite thing to do. Later they ordered a pizza and everyone had a couple of slices - their mother (my aunt) even took extra from others. I don’t usually make a fuss about food sharing, it’s nicer to let people eat, but there was one slice left and I wanted it. While we were all squished on the booth I politely asked one cousin twice if they wanted the last piece and she ignored me. I asked another cousin who was leaning on my arm twice and she ignored me too. It was awkward and embarrassing; they could’ve just said no and it would’ve been fine. They were all complaining about being too full yet the slice stayed there. The one who had leaned on me then bit the slice and they started wrapping it with tissue because they couldn’t finish it, then offered the half‑eaten piece to my sister who refused, and finally to me. I accepted it but felt humiliated after being ignored and then handed that bitten half. What confuses me is this: when I lived in non‑Muslim countries, people seemed more polite and considerate about small things like this. Since moving to a majority‑Muslim place I’ve noticed more rudeness, greed, and lack of adab even from those who pray and speak about Islam a lot. Why do many of us act like this despite the Prophet’s ﷺ teachings about kindness, generosity and respect? How can we remind ourselves that good character is part of our iman?