They do, but most people aren't aware how Internet works. Hell, most people haven't learned the concepts necessary to comprehend what information is (personal or otherwise) and how it behaves - not in terms of technology, but as a fundamental component of reality.

Anyway, the Mark as described in Revelations is pretty... bodily, for lack of better term. It evokes the image of getting a barcode stamped on your arm or your forehead, in exchange for swearing fealty. The Mark feels like a concrete, physical thing. That's why things like "government ID" or "payment chip in your arm" pattern-match to this prophesy for so many people, while things like "mobile phone number" or "e-mail address" don't.

(There's also a factor of scale/graduality. For people alive this century, countries and governments were always a thing. A big thing. Banks too. The governments, the UN, the international financial system - they look big, evil, and pattern-match to the Beast. In contrast, for most people alive today, mobile phones and e-mail addresses were something they've seen introduced gradually, from great many independent vendors. They don't have this obvious Beast-like quality.)

Source: grew up as Jehovah's Witness. While I obviously can't speak for all fundamentalist Christians, and while JW teachings don't consider government IDs to be the Mark[0], I got pretty familiar with the patterns of thinking people show around this topic.

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[0] - They do, however, believe that the Beast described in the Revelations is currently embodied by the United Nations. So if the UN ever proposes a common ID scheme or an electronic payment system, I'm pretty sure plenty of Witnesses will throw a fit.