Here is the problem, Cersei blames Jaime (at least partially) for the deaths of several of their children. Jaime also convinced Cersei NOT to kill Olenna through brutal means…Hence, not in Jaime’s interest to tell Cersei.

Don’t get me wrong, Benioff and Weiss don’t seem to care if their characters follow logical (or even internally logical) character arcs so you could be right, but it is NOT in Jaime’s interest to tell Cersei (at all).