Why isn't importing wallet.dat as straightforward as it seems it should be? In most of computerdom, an app's File dropdown menu has an Import item that permits selection of a file to import into the application.  This would seem to be an obvious feature to include in a Bitcoin GUI, yet when searching for how one imports the wallet.dat file, no such straightforward import option appears.

What is the technical reason for this?
It is not so easy to "open" a wallet, because it requires rescanning the blockchain to find the outputs that are relevant. This can take a very lock time, especially if you do not have an SSD. Because the performance is so poor, it would be disappointing for a lot of users to use this hypothetical feature.