Most office desks these days can double up as a computer desk.  Technological advancement for the computer through the years to cater for changing environments means that you can work from your lap, on the train, from your phone.  Strictly speaking the train table as suggested, when used to hold a laptop has in effect doubled up as a computer desk! 

Long gone are the early days of computers; cumbersome monitors, large keyboards and a huge computer base and hard drive.
That said, computer desks are still a necessity for home offices, especially people with limited desk space and working on a large piece of computer equipment.

Whilst laptop and notebook computers are very popular and necessary pieces of equipment they are still most largely used for business function, so it does not always follow suit that the average household will have one.