You have a very metal view of the grindcore style (It's not a reproach, I have been like that for a long time)
Peopes who are very much into the old school punk/ hardcore/ fastcore side of the thing would disagree with your view... Some despise metal music... It's how it is
Perhaps every bands were called grindcore because there weren't many bands back then...
A nice exemple of early Grind/ proto grind (Just accelerate the stuff a bit, and it's pure grind) I hear no metal influence in that stuff.
https://www.youtube.com/watch?v=caJmgYOBf14