From what's been reported, the offer was essentially 3 years, $42 million with ~$10 million guaranteed, $30M in the first two years, and some team options for two more years.
https://www.behindthesteelcurtain.co...-franchise-tag
Realistically, the only way he's not getting at least $30M is if he gets suspended for drugs again this season or suffered a career-ending injury - think worse than an ACL, I still don't think they'd cut him if he simply tore an ACL, they'd see if he could come back or at the very least they'd have to pay him an injury buyout. You can basically call that $30M "guaranteed barring flat-out stupidity."
Not signing the deal is actually a pretty huge bet against himself - to be guaranteed of at least $30 million, which is more than the sweetest guarantee you'd ever see written into a contract, all you have to do is not fuck up with drugs for this season and the following offseason.
And the outcome is even dumber - instead he took $12 million ... but guess what, the other part is the same! If he fucks up with drugs, he doesn't get any more money. But guess what else? Now if he tears his ACL he's not under contract, and the team has nothing at stake and no decision to make. He's just a free agent earning nothing. If he just has a bad year, he's not under contract, and the team doesn't have to decide whether he's worth $18M or whatever the second year of his contract calls for. He's a free agent, so their offer just goes down. Literally every other scenario now works out worse for him by not signing the offer. Maybe if it was a lowball offer he'd have a point, but it wasn't.
So either he must have such a diva-like opinion of himself that it's almost beyond comprehension, or the real reason is he doesn't like Pittsburgh and wants to leave, either of which are pretty bad signs for keeping him long-term. Of course, the third option is that he just made a bonehead move, so keep your fingers crossed that it's that one.