Probably more to do with FAA regulations than anything else. You can't put more than a 100W batter in a laptop, and the charger is 100W (96W, I think), but modern processors and GPUs need more than that at full speed with all cores, leave along fans and SSDs and screens. My similarly specced desktop runs at 400W, not including the monitor.

So yeah, we seem to have hit a limit with traditional chipsets. Probably why Apple is going full steam on their own ARM SoCs. The iPhones and iPads out now can do some pretty serious work, just imagine what could be achieved by bringing that power to the Mac.