The theoretical limits of the common one piece shafts are well beyond typical usage, sure. The problem is that a one piece shaft can fail much, much more easily and will vibrate noticeably well before a two piece design. Also driveshaft failure can be lethal on the highway. A flailing shaft can sever brake lines, fuel lines, electrical lines, etc...
I'm not saying they're a death-trap or anything, but there's a very good reason most any (perhaps all) modern Front engine, rear drive, sports cars use a jointed driveshaft. A jointed driveshaft is at least twice as safe/reliable, often more than twice.
There's also a reason why many drag strips or race tracks require special straps/shields on one piece driveshafts to try to mitigate the added risk of using that design.
Cost is the major reason why one piece shafts are popular. It's simply because they're cheaper. Aside from very high performance applications (<5% of the folks on here) the two piece design is a better overall option, IMO. Unfortunately, there aren't many good two piece options out there and there are a bunch of cheap, half decent one piece options.