The majority (by far) of the late medieval plate armor that survived is tournament armor for jousting, so basically very specialized sports equipment, designed for maximum protection and just flexible enough to sit on a horse and hold a lance. The helmet was often directly attached to the armor, so you couldn't even move your head.

They survived, because these tournaments were socially important events and rich nobles wanted to wear armor that represented their social status. When armor became more and more obsolete, these beautiful pieces were kept and passed on. The actual battle field armor wasn't that interesting to look at, so we have much fewer examples today.

People saw and tested tournament armor and concluded that knights would be completely helpless if they fell from their horse during battle. The misconception is basically like if people in 1,000 years would look at formula one racing cars and assumed 21st century people used these to pick up the kids from school. This myth became super popular, although we have many written sources about knights dismounting during battle and advancing on food. There were also tournaments fought on food. This specific super heavy armor is really just for horseback jousting and nothing else.