Because, and this is central to the point, the 70% of white people weren't responsible for oppressing black people. That's why I don't blame all white people. And why I think it's misguided to to so.
70% of Americans didn't decide to enforce segregation, or write racist laws, or enforce contract buying. Only a small minority of that 70% actively did any of these things (though many more profited from, perpetuated and ignored them). Only an even smaller minority of white people joined the far greater numbers of black people, to lift a finger to do anything about it.
If you want me to thank those white people? I'd do so gladly. But guess what, they weren't looking for black people's thanks. They did what they did because they were good people who had the courage and the decency to act when they saw injustice.
Most of the 70% that you're talking about did nothing. So while I don't completely blame them for what happened, I'm sure as hell not thanking them.
As for those 60,000 white people who marched with MLK? Well, there were 250,000 people in that crowd. So if the proportions were 70/15 (actually they were skewed more heavily to white people in those days) then maybe you can see the problem. That's 190,000 black people. Which means we're...carry the 2...826,666 white people short.
Stand by your words all you want. They were ignorant and offensive. You can think about that or you can choose to ignore it.