Given how bad domestic plane travel has become in the US, I suspect that many business trips may not be much slower even at Amtrak speeds.
The train, by contrast, involves leaving from downtown near where I live, sitting on the train, and arriving in Manhattan a few blocks from my office. It's so much simpler and more relaxing, and much easier to actually get work done during the trip, so oftentimes it's worth the extra couple of hours the trip takes. If we could invest in high-speed rail and make the whole train trip even, say, 30% faster, there would be no contest.