Why do routers/switches send ARP requests for devices that are connected to them?

I have wireshark and am analyzing packets on my network, i notice that i have ARP requests frequently.

I'm confused because i assumed once a device was attached to the network its MAC/IP address was saved in a CAM table, yet my router still broadcasts ARP messages to the network looking for the MAC/IP of my cell phone which has been on this network for 2 years.

Why does the router seem to "forget" these addresses?