Holy cow, I wrote a book!
Commenter BryanK notes that
only already-technical people (re)install Windows;
commenter Stu estimates the percentage at 99%.
That may be true in the steady state,
but the hard part is getting to that steady state.
When a new version of Windows is released,
the steady state is disrupted.
At that point, most people installing Windows aren't technical.
They're your technology columnist who is installing Windows and
viewing it through the eyes of a non-technical user,
then writing a column about it.
They're your average consumer who wants to check out
this new operating system.
These are the crazy people who stayed up late to buy
the product at the stroke of midnight,
and they're going to kick off an upgrade install
once they get home.
These initial impressions are crucial,
and innundating the user with
geeky questions they can't answer is not going to generate good buzz.
The steady state is also disrupted every year at Christmas.
The technology columnists are not as big a part of the picture,
but the non-technical end users are still around,
and Christmas is a common trigger for upgrading one's system.
So even though non-technical people rarely install Windows,
the steady state is upset every year.
What's more, whenever a new release of Windows comes out
all¹ the people who are installing Windows are non-technical.
That's why it's important for the installation process
to be as friendly to non-technical users as possible.
¹The word "all" here is about as accurate as the word "only"
in the original comment.