Because if isn’t a choice if they are told the case won’t proceed without violating their privacy further by taking phones indefinitely.
It is a choice, but like all chpices, effects come out of it.
If you choose not to give police access to a source of evidence, they do not magically get access to that access anyway. When a decision is made to prosecute a crime, it is based on whether or not there is any or enough evidence. So yes, by logical necessity, if police do not have evidence that might exist on your phoe, it is a possibility there will not be enough evidence.
That isn't a situation caused by the police or that they have any power to fix.
It is no different than if I complain about identity fraud, but choose not to give police access to my private records and online information. That is absolutely my choice. It is also very likely that they will not have enough evidence to go forward. That is not because they are being jerks. And telling me "if we cannot look at this, we may not have information that will be important to the case" is not some kind of inappropriate pressure. If they did not tell people this, they would complain, and rightly so.
It really seems like you don't understand how the police build a case or what happens in a court. You said above that the police should tell the suspect that they could be damaged by not giving them evidence. You know they do tell them that if they withhold information that they use in court, it could damage them - n fact they are legally obliged to tell them that. And you can bet their lawyers are telling them that if they withhold evidence or information that the prosecution finds it could very easily decide their fate.