The reason is aesthetics. No buttons makes it look sleek, which is all the OEMs are going for. Same reason has driven the other changes in recent ThinkPads. It's purely about looks, with making it functional coming far behind. (One example of many: New ThinkPads have no way to determine if they are charging/plugged in, except by carefully looking for a few seconds after a state transition. If the plug is loose or another problem happens while you think it's charging, they give you no notification. This is purely a twisted sense of aesthetics, trying to make it feel less IBM. And on more than one occasion, this has caused me serious issues as I discovered too late that my system was out of power.)

Have you tried "touch to click"? And also, "double-tap-and-hold-to-drag"?

IIRC, there's a setting in OS X to make it act like a traditional one- or two-button touchpad, i.e. it only recognizes clicks if your finger is in the appropriate place.

I'm not talking about tap to click. I'm talking about the whole trackpad being a button.