You know, that's the same reason I still hand-whisk mayonnaise. The motor on the blender heats things up too fast and it just doesn't set right. It's slower, for sure, but the texture is unbeatable. There's a bunch of stuff where the old, manual way just gives a better result that new tools can't match.
Totally get what @ruby310 means about the texture thing... my grandma's whisked mashed potatoes were always fluffier than anything from a mixer. Sometimes slower just is better.