There's no real need.
A great many mobile phones from the late-1990s that had pointy antennae didn't actually do anything, they were duds.
Most laptops have the antennae built into their displays, going around the edge. The only reason you'd want to add an external antenna is for attaching a directional high-gain antenna, but very very very few people are interested in doing that, and most PCMCIA/ExpressCard WiFi cards add the BNC connectors for this, so that's it really.
Adding an antenna connector does not cost 10 cents per unit: think of the design changes needed to be made to the stock internal wifi cards the OEMS buy, and other things. Things aren't that simple.
