Well, a nice 8-port metal gigabit switch will only run you about $20. True, it may be a lot harder or more expensive to do anything with port-specific VLAN tagging, but hardware support for that using something like DD-WRT is spotty anyway.
I'd also guess that the round design may play a part in that decision. It would definitely be less aesthetically pleasing to have a wagon-wheel of cables coming out of the base.
While it's silly, a cheap desktop switch is only going to cost you like $20 to get up to four ports - and it only has a 1Gbps internet port so it's not like even if you had a faster connection you're going to be wasting it.
I'd also guess that the round design may play a part in that decision. It would definitely be less aesthetically pleasing to have a wagon-wheel of cables coming out of the base.