> Why not?

It seems silly to me that an end user company not providing any network services which only has a 256 IP block has the ability to break a significant portion of the internet with a configuration mistake. There are several ways to setup dual ISPs and routing that don't involve such risk.

You need BGP and provider independent space for your two ISPs to both announce your space. What's the alternative approach?

what? this statement makes no sense from a networking perspective.

thisissue still exists if you break up your IP space, it just makes it far harder to manage.

Most places that are just doing it for that reason won't be advertising anything other than their own /24 or whatever though. You have to fuck up pretty spectacularly (and have your upstream providers do the same) to be able to accomplish what has happened here.