Copying an iPhone isn’t as straightforward as you seem to think. Copying data from a locked iPhone requires either an exploit or direct access to the SSD / memory chips on the device (basically, chip-off forensics, which likely requires bypassing the storage controllers), and I assume the same is true for Android devices.
I’m not saying such exploits don’t exist, but local police departments don’t have access to them. And they certainly don’t have the capability to directly access your device’s storage and then reassemble it without your knowledge.
Now, if your device is confiscated for long enough that it could be mailed off to a forensics lab for analysis? Sure, then it’s a possibility. But most likely if they want your data that badly they’ll either hold onto your device, compel you into sharing the info with them, or try to trick you into giving it to them. Hanging onto your data without a warrant for over a decade is a high risk, low reward activity.
Your data’s more vulnerable to this sort of attack in transit.