Where my husband worked, they would forget his birthday but not others. It always hurt him. And it was confusing too since they remembered early on in his employment, then later forgot.
I once worked at an opco location where they celebrated a coworker's birthday the day before and another's the day after mine. I got zilch, and it stung. When I still worked, year after year my birthday would be uncelebrated, while others received lunches and cakes. After a few years, I stopped caring.
I have concluded that there are those that others care about, include, and celebrate. Then there are the outsiders - they live outside of people's consideration. I'm not talking about those who are incompetent or unpleasant. If you ask anyone, they will claim to like the outsider. You see the outsider sitting at otherwise empty tables at parties. They are the ones who choose to remain at work while others party. They are always left off of invitation lists or forgotten when it comes to birthday celebrations or other milestones. It just doesn't occur to people to include these outsiders. After struggling to fit in, sometimes for years, the outsider accepts this role and becomes apathetic.