To answer the title, it's because animal fat is vilified as a killer in our country. I think everyone knows a doughnut may make them fat if you don't eat them in moderation but they do not think it will kill them. This crap is constantly brought into the office. My office mates are always complaining about their weight and/or health. There is definitely a disconnect.
I often wonder why the default snack generous people bring in to share is some sort wheat laden handful with sugar on top. No one brings in fruit, nuts, or jerky to share. There are many reasons I can think of for for that. Wheat covered in sugar is cheap. Wheat covered in sugar is convenient. To bring in the healthier options would require more money and more effort. That is what is wrong with our food system. The good stuff is harder to acquire.
I can easily say no to this crap. My addiction to it ended years ago. Part of me wants to bring in some fruit as an experiment but some of the doughnut fairies in my office will see right through my scheme and most likely get offended. Screw it, I am gonna do it anyway. I wonder how long bananas and apples will last in the common area. Maybe after that, I will bring in a small beef appetizer. I will let you know how it goes.