1) Because this can give officers probable cause to detain you. Just like if they find a hidden compartment in your luggage, even if there's no contraband in it, you'll be held many hours in a freezing cold room while they examine every detail of you and your property.
2) Because it could be a crime. Failure to disclose an encryption key if requested by UK Police and Customs authorities is a breach of The Terrorism Act, for which you can be arrested.
3) If they ask if you have hidden or encrypted data, and you lie about it, in the US you're now guilty of a federal crime -- even the hidden volume itself is empty or innocuous.
But if you are at the border and give to the officers all what they want ( access to your laptop and phone ), why would they have further suspicions to interrogate and/or detain and deep search your devices, if you are really a clean person, dunno, white, 30-40 years, suit up, good clothing, no strange stuff in traveling history..etc.?
2) Because it could be a crime. Failure to disclose an encryption key if requested by UK Police and Customs authorities is a breach of The Terrorism Act, for which you can be arrested.
3) If they ask if you have hidden or encrypted data, and you lie about it, in the US you're now guilty of a federal crime -- even the hidden volume itself is empty or innocuous.