Most (all?) TVs being sold today support HDMI, so I think the thought is that children don't have to buy a monitor too and can just plug these into their parents TV.
The popular pi distros usually support SSH by default.
Another solution would be software that finds the pi on your network. You could just use an ip scanner, but a specialized one could filter out any other devices that happen to be running an ssh server or listening on an ssh port.
HDMI connector is also a darn sight smaller, which is good for limited board space, and probably cheaper than a D-SUB connector as well. (Remember, they were pretty tightly squeezed on budget)
Maybe it's just me but it seems to me that it would be more likely that someone would get their hands on an inexpensive used monitor to tinker with instead of using the family TV. An inexpensive monitor would also allow you to set up basically anywhere in the house.
I know when I was growing up I would not have been able to tinker using the TV for very long before someone else in the family would want to use the TV.