Bacteria and yeast aren't animals. They aren't sentient, they have no perception of the world around them, they don't have feelings. That's the difference. Nearly every animal (yes, even insects) is sentient. We may not understand exactly what it feels like to be a bee (what kinds of emotions they can experience), but it's better to err on the side of not hurting an animal than assume they are mindless little robots.
Given, this is usually not the primary focus of vegan activism. Taking some food from some bees versus raising cows in the pure hellscape that is factory farming... There's a very obvious greater evil happening. Let's not let the minutia of veganism derail from the greater picture.
I suppose I just disagree with the premise... Insects ARE mindless little robots. They can react to stimuli, and have some basic behavior, but to say they experience emotions is a huge stretch. Bees have less than a million neurons, 10,000x less than a human.
If we were to follow that logic, we should keep brain-dead people "alive" on the basis that the peripheral nervous system has neurons and can independently react to basic stimuli. Thankfully doctors aren't quite so radical.
I would be VERY interested in seeing the paper on that. Because all I've read about it is that bees are sentient because they react to painful stimuli, but that's about the extent of it. To say they have feelings in any meaningful way is an entirely different conversation.
I'm no scientist, I've just read the tl;drs. It seems clear they're more than just automatas.
Obviously people can draw the line wherever they want, but the more you nitpick about what qualities make a being okay to eat or use, the blurrier the line gets. Veganism seeks to exclude all potential suffering to err on the side of caution. I mean, honey is NOT a vital ingredient to our lives whatsoever. It's not like it takes much effort to avoid.