Why does Steam insist I login for buying steam gift cards when I am already logged in?

[Phoenix]

Ok so I am logged in.
I have been logged in on this PC for 10 years. I've long since forgotten my password
I am also unable to logout to do account recovery as Steam can see me as already logged in
If I try using Steam Gift cards as a guest, after jumping through several hoops I am greeted with a blank page. This page prevents me accessing anything in Steam. I have to shut steam down and restart to get back to square one.

I am already logged in. Why can I just not make a *very simple purchese*????

[wesnef]

Likely as an additional security check, because that "very simple purchase" of gift cards is a good way of stealing/laundering money.

[Tito Shivan]

So in case you left your session open on a public or shared machine other people won't get to make purchases on your account.