Yes. It prevents hijacking of accounts.
Say Person 1 gains password access to Person 2's account (could be a spouse, lover, or other family member, or a total stranger in a phishing scam). Person 1 merges the contents of Person 2's account into a new account. Person 2's account disappears/is deleted after the merger. Bam!
Basically, I'd expect Apple has zero interest in the liability that might come from allowing this sort of thing to happen. How could Apple determine whether the person merging actually has the legitimate right to merge the accounts? The process of validating identity to a sufficient level.... nope, why take the responsibility?
In addition to this, there's the more day-to-day practical issue of, "Is merger really the right thing to do." Spouses merge accounts, then five years later it's one more thing to untangle in a divorce.