…because each character they play has a different look.
My two are Gary Oldman and Christopher Guest.
Any others?
Andy Serkis, my precious!
Russell Crowe.
I’m guessing most people wouldn’t recognize your average voice actor (Billy West, Maurice LaMarch, Bob Townsend, David Herman,) on the street.
Sadly enough, I would recognize a lot of them.
But I second Gary Oldman. The only two roles that I can easily tell it’s the same actor are his roles on Batman Begins and Air Force One.
Richard Roxburgh. Moulin Rouge, League of Extraordinary Gentlemen, Mission Impossible II, Van Helsing … I had to take my roommate to the computer and prove that they were all the same guy.
Jim Broadbent and Val Kilmer. Broadbent because his vanished into his roles, and Kilmer because he has a completely forgettable face.
John Turturro and Alec Guinness. Of the latter, it was once said that (paraphrased from memory) if he ever committed a murder the police would arrest half of England due to differing descriptions of him.
Hoo, boy, them’s fightin’ words. Kilmer’s face is in proud possession of some extreeeeemely kissable lips. The only time I didn’t recognize him was when he played Elvis in True Romance, specifically since said lips were in shadow. Rowr!
Sometimes I find that I don’t recognize Jennifer Lopez from photo to photo. Of course in her movies she always looks the same.
Sacha Baron Cohen of Borat and Ali G fame.
Gary Oldman, Gary Oldman, Gary Oldman!
Other than voice actors, that’s all I can think of. <_<
Lon Chaney.
Of course, if I meet him in the street, I’m in one of his movies.
I hang out in NYC, and the only three actors I ever saw that I really recognized were Paul Simon, Woody Allen and Johnny Depp. They all look so much like themselves that it’s scary.
Joe Pesci, but only because he seems to wear thick facial hair when he’s not doing movies and it would be hard to tell if it was him.
I have trouble recognizing a lot of them on the street. Usually I have a moment of ‘I know them from somewhere…’. Then sometimes I’ll figure out who it is or someone will tell me.
That happened about a week ago out at a bar with Laurence Fishburn. I knew I recognized him but didn’t place who it was until someone told me. I still didn’t totally believe them so I introduced myself and we talked for a few minutes, he’s a nice guy.
I go into the city frequently. It never fails that I fail to recognize someone famous until I’m at least 1/2 a block past them 
James Gandolfini, John Lithgow & Christopher Walken all come to mind; I consider each of them striking from a physical sense - whether by height or size or whatever - and I’ve walked right by all three of them.
VCNJ~
I was in an elevator with Mel Blanc once and I did recognize him. Of course the monogrammed Bugs Bunny on his shirt pocket didn’t hurt my identification.
That’s nothing. I was sitting at a bar, talking for a good 3 hours with this guy that had the most recognizable face, and I knew I had heard that laugh before. 10 minutes after I got home I realized he was one of the main characters in one of my favorite TV shows.
*It is a Canadian series so for you Canadians out there the show is Corner Gas.