[kjs3]

it's because the vast majority of people don't use SpamAssassin

Bingo. Not that there aren't a lot of people running SA, but spammers want to be able to deliver to the big players(1) (gmail, o365, etc), not the size folks out there running SA. It's not worth their time to devote effort to optimizing for a rounding error in the deliverability equation.

(1) Unless they're selling 'targeting' services where you're paying to deliver to a specific domain/user which might be behind SA. Plenty do, but that's a little bit farther down the criminality spectrum and vastly less volume than shilling peener pills or warranty extension scams.

edit: formatting