Nominally, the reason they check your bag at the airport is for security. But in the process they can find all sorts of other stuff that isn't dangerous to fly with but is illegal, e.g. marijuana. So the whole thing becomes an avenue to warrantless search.

You can't make the same argument about a computer or phone -- you can't bring down a plane or hurt anyone by having certain pictures on your hard drive. Anything you could do with a phone (e.g. trigger a bomb) would require some other physical infiltration, which hopefully would be caught by all the scanning and searching of people and baggage that we do already.

So I would hope that a sensible explicitly disallows this "airport loophole" in the process of bringing phone search into the normal legal process of warranted search... but I am doubtful. There are powerful people in the USA, UK, Australia, etc. would love the unfettered ability to search the phone of every air traveler.