You surely have a lock on your front door. Do you have such a lock on your network? Though you may think so, but it may well be wholly unlocked. Or at least, you may not be able to know for certain that it’s locked. If you use a commercial Wi-Fi router from your ISP, or one of the big names like Linksys, Belkin, DLink et al, your network may not be as secure as you think.
At the outset let me state that, as someone who reads hereabouts, you’re no dummy. You’ve taken steps to ensure that the router doesn’t still have the default admin password. You’re using modern encryption on your Wi-Fi. You’re being responsible, but there are things beyond your grasp.
The simple fact is that the firmware the runs most retail, commercial routers is closed source. As such, you have no ready way to verify it’s behavior. Yet, the manufacturer, by virtue of necessity, uses various common software modules to create their firmware. They may even use some open source modules, but end up with an closed source binary in the end.
The upshot of this reality is that you have a very small team of developers responsible for maintaining the code. That means updates come along slowly, if at all for older devices. By extension, serious security issues get addressed slowly, if they ever get addressed at all.