I wish they were, some actors just can't play the role. At least they tried casting someone physically similar to the comic book character.
Did they?
Let's take Robbie Robertson.
If your only criteria is "black", then sure, Bill Nunn covers it! But Robbie is rather tall and thin. He's a lean character, partially due to the fact that he stays in shape based on his past on the streets. They would've been better off with someone like Andre Braugher.
Bill Nunn is... not that.
Flash Thompson is a redhead himself. Joe Manganiello was not. Neither was rather blonde Chris Zylka.
Neither of those actors looks anything alike really. And these days, most adaptations make Flash blonde, like his very first appearance in Amazing.
Is skin tone the only bar? Hair color? General build? Characterization? What's the line?
The more familiarity people have with characters the more hesitant they are with radical departures.
Do most people don't really have a familiarity with MJ? Most can even describe her as a character outside of being a model and the famous "Tiger" quote. Many don't even know she was the second girlfriend, after Gwen.