I think it's a case of heraldry and Hollywood.

There are a few exceptional instances when armor was extremely heavy or did indeed render its wearer almost “locked” in a certain position, such as armor for certain types of tournaments. Tournament armor was made for very specific occasions and would have been worn only for limited periods of time. The man-at-arms would have mounted his steed with the aid of his squire or a small step, and the last pieces of his armor could then be donned after securely sitting in the saddle.

This notion appears to have originated during the late nineteenth century as a joke. It entered popular fiction during the following decades, and the image was finally immortalized in 1944 when Sir Laurence Olivier used it in his movie Henry V—despite the protestations of his historical advisors, who included the eminent authority Sir James Mann, Master of the Armouries at HM Tower of London.

Worth noting - infantry armor and cavalry armor ain't the same and that which will allow you to survive being hit by a lance is different from that which will allow you to survive the battle of Agincourt.