I think there are two factors causing developers to choose too-long animations:
1. Pride. We worked hard to build this animation, and the user is damn well going to watch it! Because they'll definitely like it as much as we do. (This is an example of not putting the user first.)
2. Inaccurate conclusions from user testing. I think users go through two phases. Early on, they don't know their way around the UI and can't navigate quickly. Plus the animations are novel and they're still enjoying them as eye candy. So for two reasons, they don't mind slower animations. As they use the UI for days or weeks, both these factors flip. You start navigating the UI by muscle memory, and novelty is gone. Thus you can only find the optimal animation speed by testing how a user feels about it after weeks of usage, which isn't how user testing is usually done.
1. Pride. We worked hard to build this animation, and the user is damn well going to watch it! Because they'll definitely like it as much as we do. (This is an example of not putting the user first.)
2. Inaccurate conclusions from user testing. I think users go through two phases. Early on, they don't know their way around the UI and can't navigate quickly. Plus the animations are novel and they're still enjoying them as eye candy. So for two reasons, they don't mind slower animations. As they use the UI for days or weeks, both these factors flip. You start navigating the UI by muscle memory, and novelty is gone. Thus you can only find the optimal animation speed by testing how a user feels about it after weeks of usage, which isn't how user testing is usually done.