“All lives matters” is a complete dismissal of the point.
Imagine that you go out to dinner with your family, and everyone gets what they order except you - you get a plate full of dicks. You say, “Hey, I should get what I ordered!” And your wife responds with “Everyone should get what they ordered,” while chowing down on the food she ordered and doing absolutely nothing about your plate of dicks. The effective meaning of that sentence is to completely ignore your complaints. Yes, obviously everyone should get what they ordered, but most people already do.
Or, to put it another way: when I say “black lives matter!” I am specifically addressing an inequality in the system: the fact that black lives, by and large, don’t seem to matter, at least not in the way white lives do. If your response is “all lives matter!” you have at best missed the point and at worst are trying to deflect from it. Yes, of course all lives matter, but right now we’re trying to pay special attention to black lives because that’s the point where “all lives matter” seems to be breaking down. It’s like wondering why there’s no such thing as a straight pride parade - you completely miss the point of the exercise, and at this point in the conversation it’s entirely reasonable to assume that you’ve either just stumbled into the room for the first time, or you’re doing it on purpose.
This is what happens when you blindly play semantics with absolutely no attempt to understand the context; the equivalent of wondering why they’d need a plane in wood shop because you couldn’t fit a Cessna through the door.