I think they're called that because they postdate the "looter shooter" that combined Diablo-esque "action RPGs" with FPS games, like Borderlands and Destiny. "Looter" without the "shooter" is a much better name for Diablo's genre anyway, since we have far too many RPGs that are also action games and have nothing in common with Diablo.

I'm still waiting for the resurgence of the style of shooter that came just after those that inspired this wave of boomer shooter; the likes of Half-Life, Halo, 007, TimeSplitters, and so on. I don't know what subgenre will be assigned to those games when they start to come back around, but that style is also old at this point, so hopefully it doesn't also get assigned the label of "boomer shooter", because then it'll be harder for both audiences to find what they're looking for.