I might BU with this as it says more about the parents, but...
When I was a child, I was taught to say thank you to everyone who gave me a gift or money for my birthday. This included picking up the phone and saying thanks to people who sent money in cards. I'm in my early 30s so this wasn't that long ago, really. When I was too young to do it off my own back, my parents helped and reminded me. Some of my relatives would, and did, stop sending money to children who never said thankyou. Maybe that's why I get annoyed if I don't get a simple thanks.
We've given birthday money to 4 children in the family in the last 2 months and haven't had a thank you from one. This is basic manners and it irritates me.
WIBU to not do it in the future? Money doesn't grow on trees and saying thank you isn't hard (even copying and pasting a generic text takes 1 minute). We are in a cost of living crisis and there's better things I could have done with the £80 total - not a lot to some but two tanks of fuel for me.
Happy to accept AIBU because parents should be raising children to say thank you ultimately.
I am possibly more annoyed by it because the parents themselves are very expectant and entitled when it comes to this sort of thing (people spending their money on them).