I think I understand your question, and reworded the headline accordingly.
If I follow you, you:
Went to add a child but didn't know that the child already had a WikiTree profile.
After submitting the child as a new person the system suggested that the person might already exist.
You accepted one of the matches.
You're asking, since you accepted the match, why didn't the system automatically set that person as the child.
I agree, that would be a nice feature, if you are on the Trusted List of that other profile, as Becky points out.