[claudius]

Probably because by the time it became usual to plug them in often (read: portable computers), they were already commonplace to a degree that made it impractical to replace them.

And nowadays there’s Wifi/UMTS available for most such usecases – I have a docking station at home and use the Thinkpad’s internal Ethernet port maybe twice a year; mostly relying on wireless data transfers when not at home.