Can someone explain me why some footage looks like it's interlaced, but it cannot be deinterlaced. I mean, when trying to deinterlace it, the output is two "identical" frames with just some garbage in finest details in the second frame.

Is it just because it has already been deinterlaced or scaled from sd to hd with absolute shit quality filters? This happens a lot in todays HD channels which shows reruns of programmes produced early 2000's.