Who is making this claim, in relation to players and their play of their PCs? And what are their examples of it (either RPG texts that suggest it, or actual play that exhibits it)?
I mean, here is an early example I know of, from Gygax's DMG (p 93), discussing player contributions to setting/backstory; it is in the context of building strongholds:
Assume that the player in question decides that he will set up a stronghold about 100 miles from a border town, choosing an area of wooded hills as the general site. He then asks you if there is a place where he can build a small concentric castle on a high bluff overlooking a river. Unless this is totally foreign to the area, you inform him that he can do so. You give him a map of the hex where the location is, and of the six surrounding hexes. The player character and his henchmen and various retainers must now go to the construction site, explore and map it, and have construction commence.
The player didn't propose
a high bluff overlooking a river, suitable for the construction of a small concentric castle because he thought that this was
the most likely terrain. The player wants his PC to build a castle!
Relating to the thread topic, it seems that Gygax was less conservative about the role of players in contributing to setting/backstory than some of the posters in this thread who frame their expectations for play in terms of D&D.