Searching a digital device and further remote databases it has access to is more akin to going to the persons home and riffling through all their papers and interrogating their friends and family than it is to a traditional customs search.
It's reasonable to search for harmful or illegal materials entering the country. You have a compelling interest in the safety of the citizens and this is likely your one chance to reasonably interdict them before they can cause harm to the citizenry.
No such special circumstance exist for information. Information flows freely and securely over most of the world and interdicting it at the border is laughable. Once you leave behind existing powers and arguments for same you are left making an argument for new powers undreamt of by the founders.
You can argue for example that being able to access all the private words and documents of travelers allows you to make better decisions as far as whom and what ought to be interdicted or examined. This might even be a compelling argument for you. It is however not a continuation of centuries of practice but rather entirely new power that ought to be enumerated in new laws if it is to be granted.
If we insist that we react to the changing landscape provided by technology solely by interpreting what 18th century individuals wrote we ought to interpret very conservatively insofar as grants of government power. If such new powers are needed let the peoples representatives write the required laws.