Because you show up with nothing on you, and there’s no way to prove that the backups even exist. Especially given how often people travel with blank/disposable phones.
Just as the border guard can’t require you to fetch something from your house before entry, they can’t require you to restore from a remote backup that they don’t even know about.
Wiping under duress is
unlawful and could lead to prosecution in some juridictions.
On the other hand I don't know of any juridiction that force you to carry all the personal data in a single device when crossing borders. It would moat likely not even be possible.
Just as the border guard can’t require you to fetch something from your house before entry, they can’t require you to restore from a remote backup that they don’t even know about.