Because it's an AT$T issue?! Be real, it's a network issue.
I have a 1st gen iPhone and almost to the day the 3g phone came out my bars in my house went from 3 bars to 1 bar and lots of dropped calls and delayed voice mail. AT$T pretends it's not an issue but they continue to put up those damn signs saying "More Bars in More Places." I'm starting to think they're referring to liquor establishments!
The 3G problems were fixed in 2.0.2, that is why people in rural 3G areas have no problems, in urban areas, idiots have refused to upgrade to 2.0.2, or they have jailborked using 2.0.1 as a template. Upgrading to 2.1 will fix most of the 3G and other iPhone issues.
TEG