It was specifically designed for the wearer, and fitting perfectly distributed the weight very well, and allowed the knight to move easily. Indeed, someone recounted that one French knight at Agincourt would routinely go for a run of several miles each morning in his armour. Unfortunately, the suction of the mud at Agincourt had a tendency to stop men in plate armour getting up if they fell, leaving them vulnerable to the (peasant) soldier’s best friend, the heavy hammer.

By contrast, my understanding is that a lot of costume armors for TV and film are made of weaker, lighter and cheaper materials

Well, what they can do, man… Hollywood is notoriously impoverished for decades. ;]

Now the funny thing is this material problem shouldn’t apply to video games at all. After all, video game armors – any armor that exists only in CGI – doesn’t have to bother itself with the rules for this or that material. Thus there’s nothing stopping CGI artists from making armor with articulation, scales and so on.

Which narrows it down: ruin is in the heads, as usual.

The final factor, I think, for science fiction armors is an aesthetic one

So there are reasons conjectured to be relevant in some cases, even though the problem remains exactly the same outside that specific park. Occam’s razor? Nah, there’s too much smoke to use this mirror for shaving anyway.