I feel really hurt and confused about why my dad would keep something this important from me

Trigger warning: death/loss/grief/serious illness

Hi! 💖 The story that I am about to tell involves the death of a family member due to a serious illness. If this is a sensitive or triggering topic for you, please feel free to stop reading at any time! 

My parents have always made a big emphasis on full transparency and being honest with them about everything that's going on in my life. Lying - even lying by omission - was not tolerated when I was growing up. When it came to me, the concept of "this is something very personal or embarrassing that I would rather not share" did not exist when I was a child. 

When I got older, I started to notice inconsistencies - my parents were definitely not returning the same full transparency. There were secrets they kept from me and from each other. There were secrets that my mom kept from my dad. There were even more secrets that my dad and my grandmother kept from my mom. We all lived together (my parents are not divorced), but after I spent a day with my dad and my grandmother, I was expected to not say a word about it. I was not allowed to re-tell where we went, what we did, what we saw, etc. Saying anything - even something as innocent as "my grandma got really cute new shoes!" - was a potential reason for my dad and grandma to call me a "loose-lipped idiot who cannot be trusted."

A few years ago, my parents and I moved and my grandma stayed behind. She and I only saw each other during Skype calls after that. Three years ago, my grandma and I got into a seemingly minor argument, but she got really upset with me, accused me of being disrespectful to her on the most difficult day, and hung up on me. I had called her many times, wrote at least five messages telling her that I had no idea that she was having a difficult day, and that I am genuinely sorry for anything I said that may have come across as disrespectful because I never meant to be rude. I wrote her two letters in 2022 as well, due to some current events and because I was worried about her safety. She has ignored all of my attempts to reach out, but continued to talk to my dad. 

Two years ago, I was told that my grandma was dead. My dad told me that she had a heart attack and because she lived alone, no one had been able to call an ambulance in time. He had told my mom the same thing. I hadn't been able to go to a memorial service for her due to previously mentioned current events (I'm doing my best to keep politics out of this), which made my already existing feelings of grief that much more complicated. Two days ago, I found out from my mom (who found out from a friend who helped with my grandma's memorial service in our absence) that she never had a heart attack and that she really died from cancer. 

I'm having a really difficult time with processing all of this. I feel hurt that my dad would lie to me - if I was old enough to understand what a heart attack is, I was old enough to understand what cancer is. Why would he lie to me? What was the point? But what's even worse is, this wasn't something sudden that no one was prepared for. He knew that she had cancer (he admitted it). She knew that she had cancer. They both knew that it was terminal. And not one of them has thought to tell me, to at least let me say goodbye. I am just really struggling right now!