Because your friend would just be doing it out of curiosity, not as part of an investigation.

It doesn’t matter why my friend wants to use my phone. If a friend wants to use my computer, I log out and sign into a guest account that doesn’t have access to my private documents. That’s not an option on my phone. It doesn’t matter if my friend has a legitimate reason to want to look at something on my phone. Maybe they want to see a picture I took last Friday - if they tell me that, I’ll just pull it up and share it with them.

If my phone has something on it that can help the police and the police tell me what they’re looking for, I can check my phone myself and share specifically that information with them.

If my phone doesn’t have that information, I can tell them that, too.

This is the exact same as with my friends. The difference is that the police are much more likely to be antagonistic and much less likely to tell me what they want.

If the police can’t articulate what they’re looking for or if they don’t trust me to tell me what it is, then I DEFINITELY don’t trust them to look at my phone themselves. And heck, that’s true of my friends, too.

If I hand a police officer my phone unlocked, what’s stopping them from hooking it up to GrayKey or Cellebrite or some other similar tool, and dumping all the data from my phone without my knowledge - whether for legitimate or nefarious purposes? What stops them from doing this “out of curiosity?” This isn’t generally a risk with my friends, but it’s always a risk when dealing with the police.

In the US, when there’s suspicion of police wrongdoing, the police investigate themselves (and either conclude that nobody did anything wrong or that only one person did something wrong but everyone else is fine). It’s so bad that it’s a meme (“We’ve investigated ourselves and found no evidence of any wrongdoing.”) But even if you don’t have the police investigating themselves in your country, it’s still the government doing that investigation. And nothing makes the government inherently trustworthy.

As a private citizen of your country who was legitimately concerned that the police are retaining more data than necessary, could I visit the police station and ask them to give me supervised admin access to their computers (as well as the personal computers of anyone who might have had access to my device or to the data extracted from it), as well as full access to the station itself in case there are any unaccounted for computers, so that I can confirm that the police aren’t overstepping? If not, why not? It’s not like the police have anything to hide, right? And the sooner that the police cooperate and that information is shared with me, the sooner I can rest easy knowing I and my fellow citizens have not been victimized by the police.

Hopefully you see how ridiculous it is for me to expect someone to just give me access to all of that information. That’s actually less ridiculous than a police officer asking me to hand them my unlocked phone.

As a private citizen, I have to trust that police and government officials are doing their jobs properly. If they don’t, I can have my privacy invaded or be framed for a crime, with no method for recourse. And without any real accountability. I have to trust a police officer if I hand him my phone, and I’m the only one risking anything. In the opposite scenario, if I overstep while they’re supervising me reviewing their systems, they can hold me accountable immediately.