Well, for one, those who saw him in other photographs or films captured that day. Those altering the Altgens photo didn't know what else was out there. They had to be concerned about not contradicting other images.
And, notice that Doorman is entangled with the Black Man in the Altgens photo. I don't believe it looked like that originally, but regardless, it would not have been easy to blacken out Doorman alone. It's very presumptuous to assume that it would have been easy to take him out.
If any part of Carl Jones was captured in the Altgens photo, it was his left arm and shoulder, as we see at the start of the Wiegman film, which was taken at the same time as the Altgens photo.
That isn't just theoretical. That division between "blind" and "see" is indisputably true. But obviously, I don't necessarily have Carl Jones in the right size, and it's possible, even likely, that part of his left arm and shoulder were exposed in the "see" zone and captured in the Altgens photograph. And if so, it would have been very difficult to piece around it to just remove Oswald. And if they removed both of them, again, how would they reconcile that with other media? Carl Jones was captured in both Wiegman and Hughes films, that I know of.
And since Black Tie Man, next to Doorman, wasn't there either, it would have resulted in this:
That isn't bad, but realize that digital tools make it easy, which did not exist back then. And again, if other media showed Oswald and Carl Jones, as would be expected, then they'd have a lot of 'splainin' to do. And considering that you're still left with a faceless guy who is visoring his eyes which are supposedly ensconced in shadow as dark as a coal mine at midnight (and yet, he has the need to visor), and a guy with his arms, both arms, folded across his chest in a restful position, and yet there's this white tarantula sitting on his face (that some call an elbow, in which case he's got three of them) it is hardly a pretty and reassuring picture.