Because if your home IP hasn't changed (and mine doesn't regularly, often for months and months it is static), then the old lookup will still be successful. The Access Key system is a dynamic IP lookup system, much like Dyn.com, but only for MCWS (the Media Center API).
Even if your WAN IP does change, your ISP is probably serving your "network neighborhood" from a local pool of somewhat limited size. It wouldn't be that surprising to get the old IP back again at some point in the future. And, of course, this also assumes they aren't on your local network. If they're on your local network, the chances of regaining the same internal IP are even higher (and if you do have port forwarding opened, then you almost certainly need a static IP on your LAN anyway). Not that it matters, because they could just scan the network for the service (MC itself will auto-discover servers on the LAN).
If you don't want people to get into your Library using your old Access Key, then make sure Authentication is enabled and change the Username and Password (if they have the old one).
That won't stop someone determined, but it should stop any casual onlookers. If you want it to be truly secure, you'll need to run a VPN Service at home to access your Library only through the VPN.