In reality it doesn't default these folders to the C drive.
It defaults them to the drive that Windows is installed on.
If you were to install to the D drive that would be the default for Documents and all the rest.
As for defaulting to the drive that Windows is installed on, that is done because it knows that drive exists and is usable. Now for sure they could ask during the install of Windows where you wanted these folders, but most people don't want to change it and as such it would be adding "extra complexity" to the install for most people that don't need it.
Not to mention it gets even more "complex" is you are talking about upgrading of the operating system. Just because you another drive with lots of space on it doesn't imply that you want to move your personal files to it. So again another series of questions and code to do something that most people won't need.
And note the only time you would know how much space these folders take up is for an "upgrade", and even that doesn't tell you anything about what might be put in them in the future.