Why are people so insensitive about death?

One of my peers said, “It doesn’t even matter anymore. The selfish vain woman is gone and she never cared enough to be in your life. Why grieve over a woman that didn’t want you? She is DEAD. Get over it and move on! Not everyone has to feel sorry for you!”
I know my late mother walked out on my dad and I but it doesn’t mean I can’t grieve over her. My dad called my late mom a “bitch” right in front of me because of what she did. He didn’t care if I heard it and she was selfish and a pathetic woman that left because they have a special needs daughter.
Ouch that hurt. I’m like, “Hello?! I can hear you!” He said, “Good! I wanted you to hear it. You need to hear it so quit talking about it. Your mother is dead and she isn’t coming back!!!!” He sounded very upset and hurt too. He can be brutally blunt at times. I still miss my mom. She passed last January. 💔💔💔💔😭😭😭😭😭😭😭😭😭😭😭😭😭