If Tammy's actually playing a long-con to cash in on some life-insurance check by feeding Jack to death, she could have done the deed 10 years ago. By this point, their finances are fucked, and no insurance company wants to take an obese quintuple-stroke fake-Italian.
That, and Tammy was the breadwinner with her old job as far as I know. She couldve kept her job after leaving Jack, and probably made than if she is fishing for a pay day with a Life Insurance company–and be happier anywhere else.
While the idea of Tammy Sr. being the world's most patient black widow got a smile out of me, I think the explanation is much, much simpler: she's the pillar that holds the Scalfani home, and under a lot of social and religous pressure that she can't seem to escape.
Despite all of Jack's apparent god-bothering, he's not all that pious; what little we know of his pre-Tammy days doesn't show much evidence of overt religiousness, and he barely bothers with mealy-mouthed lip service nowadays. But we do know that Tammy used religion as a cudgel to make him tone down his drinking. I think she's the actual religious person in the household, and the rest of the family goes along with it to receive gud
mit boi points/not get yelled at. And, as a Good Christian Woman

, the very idea of divorce is something she simply can't abide; "in sickness and health" and all that.
On top of that, they're living in a conservative state, have made boomer-grade conservatism/democrat oppositionism part of their core identity, much of their circle of friends and acquaintances is church-related and, finally, there's the moral issue of abandoning a morbidly obese cripple, who has a laundry list of health issues and more bacon cookers than working brain cells, to his own, well-deserved, fate.
She probably considers the consequences of breaking any of the tethers binding her to Jack to be severe enough backlash (breaking vows/sinning, losing friends, family and community, being shunned by society), that she doesn't even try. Apathy has set in and, consciously or not, she's letting Jack eat himself to death as the fastest, less guilt-ridden path to a normal life.
TL;DR: Tammy lives in a society.