Except that most (not all) routers do not have physical separation between their networks and thus it is possible for intruders to jump from one to the other. The Ubiquiti EdgeRouter X is one that does have physically separate networks, as does (I understand) the Netgate SG-1100. The former is apparently quite complex to set up (nerd-friendly), and Ubiquiti has been bitten by some bugs recently.
The first answer here mentions security as an issue when using a single router, but is not considering the hardware that I have mentioned in the previous paragraph. Other answers go into a little more detail about why to use separate routers - in short, most home routers are not designed to provide separate networks that are secure from each other.