Just taking a look at things from Smart's perspective, if he's being honest with himself...
He's been on this team since our roster was garbage. Now, over maybe 12 million a year vs 9 million a year, he'd consider leaving this team to likely go somewhere much less competitive?
He isn't a typical 12m a year FA. This Celtics team mitigates his weak shooting, and on this team his winning plays and hustle have the maximum possible effect. It's almost a perfect fit.
He might get paid a few bucks more elsewhere, but he's the ultimate competitor, and I have to think he factors in that playing here allows him to be the best, most impactful player he can be.