I am now.

A passing thought on something Delia mentioned in her (as Kent said very astute) comments below. Is there a simple explanation for why (1) is so much more acceptable than (2)?

(1) I am here.
(2) ??I am now.

Off the top of my head I can’t even tell whether this is basically a syntactic, semantic or pragmatic phenomenon. As with most things I scribble down here, I assume there’s an answer somewhere about this.