In Collateral Beauty Time, Death & Love are actors who were hired by Howard's employees.
But in the final scene, these 3 actors are still only visible to Howard but not to his wife. Why is this so?
When Hiring those three "actors" for the job, Brigitte asks "what are the rules" and then she clarifies that she is asking for the "rules of the theatrical device" followed by "Do other people see us or only Howard?". After a few rounds of discussion and argument, She herself says "We have the power to be seen by whomever we want, whenever we want."
In the end it is shown that Brigitte was actually death, meaning what she said about the rule was true. They chose to show themselves to Howard only, in the last scene.
The real time, love and death are only visible to Howard because hes the person they've chose to help, so no one else can see them.
Actually, it's not 100% clear they are visible even to Howard.
Within the reality of the movie, they are visible to whomever they need to be visible to. For example, Brigitte must be visible to both the private investigator and her grandchild, since they have to pretend to not see them, and apparently all three appeared originally on the video, or else the technician responsible for erasing them would have mentioned it.
It might be they are appearing to Howard for the purpose of reminding him that his friends did not really betray him, or it might be they are just looking over their handiwork.