I am quite sure you will continue to believe what you believe, but I will just point out that I think you are confusing focus with exclusion.
All lives have always mattered, a statement that should not need to be made. It's an inherent property of respect for life.
But if, as many believe, the category of "all lives" has too often excluded black ones, then the statement that "Black Lives Matter" has a specific purpose. But that purpose is within the idea that all lives matter, not against it. To counter it with that otherwise needless statement is to imply that it is contradictory, conspicuously to deny its truth or validity, and that is, to many people, a racist response.
Go ahead and disagree, but understand why not everyone does.