The cops have one job—try to arrest you and build a case to convict you. Why would you make that job easier??
Let’s say there was a bag of fentanyl waaay under a seat that was lost there before your dad bought it. Congratulations, your dad goes to jail for years.
Let’s say the cop is pretty sure there might be contraband in the car. He can rip the seat cushions apart, pull the dash off, dismantle the engine, and when he finds nothing say, “you’re free to go”. Oh, and they won’t pay to fix your car because you gave them permission to do that.
On your phone, let’s say you have a friend that’s being investigated for a crime and he venmoed you $500 for a laptop you bought from him. Great. Now the cops can charge you with conspiracy by saying it is your share of the crime. You’ll have to hire a lawyer and go to court.
On your phone, let’s say some porn site emailed you spam and one of the photos attached is an underaged girl. That’s child porn.
On your phone, let’s say you have some embarrassing nude photos. Do you want them pawing through them?
Let’s say that on your phone there’s an email receipt from a locksmith supply store. You didn’t know this but in your state what you bought is legally “burglar tools” and is illegal. Now they get a warrant to search your house.
Or, and here’s a fun one, let’s say the cops are looking at your phone and ask you “where were you on the night of February 22nd?” and you say “I was at home.” but Google Maps says you were in the next town over. Congratulations. You lied to the police (which is a crime) and you lied about your whereabouts and there is evidence placing you at the scene of a crime (which you are unaware of), so they can arrest you for lying and now have probable cause to arrest you and search your home.
There’s two outcomes to a police interaction where you’re a suspect:
1) you are arrested and charged which might ruin your life, which is what they want, or
2) you are free to go, like you were before they stopped you.
Why make their job easier to ruin your life?