My thought is that they're doing all these other things first. Developing the gameplay system, mapping out the areas, figuring out puzzles, writing another mediocre story (well, in the case of DoS and PoR), etc. etc. And then, towards the deadline, they're all like "OH CRAP. Did anybody draw any new sprites?!" "Son of a..!" Yeah, that. Quite a stretch but it is possible.
Another question is, why draw out all these new versions of Dracula and such, who's a much bigger character, and not even bother to at least give someone like Richter a new outfit at the most. I know it's possible. A lot of people on here are doing it, and if they actually worked on it, they could finish it in a couple days. They have a team of spriters, after all.