> The job of a router is to be able to receive multiple packets from multiple sources then send the packets 1 at a time in each of its outputs.
No. The job of a router is to route packets between networks.
> If the routers are able to do that then why is there congestion at all?
Suppose you have a WAN router with a 1 Gbps LAN and a 100 Mbps WAN. When routing from the LAN to the WAN, the router can receive up to 10 times as much LAN traffic as it is able send on the WAN. That is congestion, and the router could be forced to drop most packets received on the LAN destined to the WAN.