Cause you're not capturing 60i. Not really. Your signal is still 30p (assuming you set your output to that) psf means "progressive segment frames" and is just a way to transmit progressive material over an interlaced pipe.
So while the signal may seem like it's interlaced it's actually progressive, and once captured properly is indistinguishable from a signal transmitted in a progressive pipe.
Most video signals (even progressive ones) are transmitted as psf. Even in professional environments.
Also there isn't a "massive" difference in quality between 1080i and 1080p, but that's not an argument I'm inclined to get in to.
Like I said, feel free to spend your time trying to get the DVI signal from your graphics card to be P instead of PsF. I don't know how to do it, or even if you can. But I do know, if what you are outputting is a PsF signal, you're not going to gain literally a single thing from all your efforts.