Because it's not a copy of the cartoon. That's why. I saw Transformers two not long after it came out. I'm not a fan of the cartoon but I enjoyed the movie. Yay robots. Yay explosions.
I have a friend who's been a Transformers fan since it came out in the 80s and he would NOT FUCKING LET IT GO. He didn't even SEE the movie and kept on and on and on and on and on about how wrong it was. To the point of when he finally did see it out of spite he was so tainted with hatred for it he couldn't enjoy it.
Is that how people want to see something? People are going to enjoy this movie, "whitewashed" cast or not. People that have never even seen the show and won't know any better. Are they all racist? Are they all stupid?
Does it suck that people can't just get a job no matter the race or color? Yeah. But it goes all ways. And even when movies or shows make a conscious effort to diversify a cast people still complain about the portrayal. No one will ever be happy.