It's because people disagree with your assumption that you are completely correct, but I know you're already not here to discuss but to "fact-check".
There are reasons why specific pieces of history stick out and resonate. His "I have a dream" speech is what I remembered most because that is what ultimately, I believe, actually played a role in truly removing a lot of pervasive racism and discrimination that used to exist.
It certainly helped me from a young age really have a worldview that people are who they advertise themselves as, not by the visual identity I associate them with. It's never been an issue until people like you popped up out of nowhere saying I'm the most racist person alive because I don't treat people differently because of their race.
MLK could have thought any number of great or terrible things. In some contexts it can be highly appropriate to mention them.
In my opinion, when someone mentions MLK in the context of the "I have a dream" speech, its clear to me that the personhood of MLK or his beliefs are really not up for discussion. MLK is a piece of iconography, a historical reference point, for the color-blind, racially-neutral position. Any discussion of MLK's beliefs distract from the point being made and ultimately does not change minds.