One big difference is that digital searches aren't limited to stuff you're bringing with you -- they want to search your entire past history in terms of email, social media, etc.

Another big difference is the purpose of the search in the first place. We search for physically dangerous items to protect people, or unlawful items for customs reasons. The case for physically dangerous data and unlawful data is much less clear cut.

The real reason for searching your devices seems to be related to profiling you as a person in order to determine if they want to let you in, which is a completely different purpose than searching your bags.