Q. Why didn’t Isaac confront Jacob about stealing the blessing meant for his brother Esau?
Your question is about the account in the book of Genesis of how Isaac’s younger son Jacob tricked his father into giving him, rather than his brother Esau, the blessing that Esau should have received as the older brother. Readers in many contemporary cultures will have questions about this account because in it, the authority figure (Isaac, the father) does not act in the way we would hope and expect authority figures to act.
For one thing, as you suggest, from the perspective of many contemporary cultures, Isaac should have confronted his son about his deception and theft and corrected him. Beyond that, many contemporary readers will wonder in the first place why, when Isaac realized what had happened (“Your brother came deceitfully and took your blessing.,” he told Esau), he did not retract the blessing that Jacob had obtained fraudulently. Was it really the norm in this culture for people to be bound by their word, even if they had been led to give it under false circumstances?
Apparently so. There is a comparable account in the book of Judges of how the Gibeonites, a tribe living in the land of Canaan, deceived the Israelites into swearing an oath of peace with them by pretending to be a group that lived far from Canaan. The Israelites were supposed to destroy all of the Canaanite tribes, but when they finally learned who the Gibeonites really were, they said, “We have given them our oath by the Lord, the God of Israel, and we cannot touch them now.”
As I said, this may seem strange to many contemporary readers. We do not consider people to be bound by their word if they have made a statement under compromised circumstances. In the United States, for example, a confession can be dismissed as evidence if it can be shown that it was made under duress. What readers of the Bible may wonder most is how God could consider people to be bound by their word under such circumstances. Isn’t God fair? Why would God hold people to statements they would not have made if they had not been deceived?
I think the answer, as we see often in the Bible, is that God chooses to work within the conventions of human cultures to pursue his redemptive purposes. The Bible clearly disallows many cultural practices that are destructive of human flourishing. But in general, as I have said in other posts on this blog, God works out his plan through the free choices, good and bad, of human moral agents, accommodating human cultures in the process. Rather than completely setting aside the cultures humans have built, which are often for the most part positive creative achievements, God looks at a situation and says, “I can work with that.”
But this brings up an important interpretive principle: As one of my seminary professors used to say, “Narrative is not necessarily normative.” Just because Isaac, based on his own cultural norms, considered himself bound by a blessing he had given under false circumstances, that does not mean that we today should enforce the same norm. Rather, I think that based on the counsel of the Bible overall, we should only hold people to their word if it was given fully informed and with free consent.
So to answer your question, I would say that Isaac did not confront Jacob about stealing Esau’s blessing because Isaac considered it a “done deal” according to his cultural norms and there was nothing he could do about it. But we do not need to take that as a model for ourselves today. I think we should instead encourage people who have been led to give their word under false circumstances to take back what they have said and not consider themselves bound by it. And yes, they should confront the person who deceived them and impose any consequences that would be appropriate as a penalty and correction.
Good Question 
“Just because Isaac, based on his own cultural norms, considered himself bound by a blessing he had given under false circumstances, that does not mean that we today should enforce the same norm.” Seriously?!
Yes, seriously. The Bible as a whole does not consider a blessing, once spoken, to be binding if circumstances turn out not to be what the person speaking the blessing understood them to be. For example, Jesus himself told his disciples when he sent them out, “Whenever you enter a city or village, search for a worthy person and stay in his home until you leave town. When you enter the home, give it your blessing. If it turns out to be a worthy home, let your blessing stand; if it is not, take back the blessing.” More generally, the Bible as a whole does not consider that words, once spoken, must be allowed to stand. The Bible records a variety of approaches to this issue. For example, in the law of Moses, in the book of Numbers, it says, “If a young woman who is still living in her father’s home makes a vow to the Lord or a pledge under oath, if her father hears of the vow or pledge and does not object to it, then all her vows and pledges will stand. But if her father refuses to let her fulfill the vow or pledge on the day he hears of it, then all her vows and pledges will become invalid.” Presumably this law was intended to protect the young women of this culture, who would not necessarily had the knowledge and experience to make sufficiently informed vows and oaths, from the consequences of inadvisable verbal pledges. Once again, the principle is not that words, once spoken, must stand. From the variety of examples in the Bible, we must strive to understand the full counsel of God.