Although it might be convenient for wallets, it requires a reader to examine the contents which might be inconvenient for the police who might want to challenge the carrier. Also, a magnetic memory could be easily altered for nefarious purposes.

And lesser off ports of call would need the equipment to deal with the cards. The crruent paper/stamp is much easier for them to deal with.

Now a combo smartcard/book would do for both, provided you coule eliminate/reduce fraud on the electronic side of things.

It's called an ID card. Most countries in Europe issue their citizens with a national ID card which certifies citizenship. You can use this to travle between most countries in Europe, You only need to get a full passport if you travel to countries which demand a conventional passport.

That would be very useful if you only planned to go through US borders to nations that accept the card system. When traveling to many other countries they still stamp visas in your passport, so you still need the paper pages. The card system would have to be implemented by every nation simultaneously.