> They haven't changed due to the US embargo for decades. So what is the point?

Precedent, I assume. If you start an embargo or other sanctions, and then drop it when the entity shows that they're not going to back down, you're sending a signal to others: show that you're not going to back down and we'll drop the sanctions because what's the point? And that will dull the blade of the threat.

It's similar to negotiating with hostage takers and terrorists. Once you do it, you encourage everyone to take hostages in order to "negotiate" whatever they want done.