all lives matter isn't offensive on the surface either, but it's usage is a reactive/dismissive stfu to black lives matter. if people can pretend blm means 'black lives matter more', or 'only black lives matter' then they get to pretend to the offended party, and to be the more reasonable inclusive person by replacing black with 'all',

but these people aren't watching a st judes cancer kids commercials, or ukranian refugee news clips and saying 'omg that's terrible. all lives matter', they only ever use it as a way of saying fuck blm

white lives matter is just a more extreme pointed version of that.