Quite simply, I think that Angel has spent so much time looking down on Spike (disregarding the obvious height differences) that he has never actually seen him. Spike is the one who stayed with and protected Dru, even after she gave him numerous reasons to leave her. Spike is the one who stayed in Sunnydale and protected Dawn and Buffy's friends between Seasons 5 and 6, even though Xander, at least, never ever really trusted him or believed his motives. Angel has felt contempt for him ever since Dru first brought "Willy" home to meet the family, and that contempt nearly cost him his life in Season 5 "Destiny," when he totally underestimated Spike, as usual, and almost got staked for his stupidity.
Most likely, because Angelus never cared about anything but himself and never served anyone's interests but his own, and always saw Spike as just some young pup trying to be like his granddaddy, Angel has retained that image, and never bothered to notice that that isn't what Spike is anymore.