If devices are connected to the same switch but are in different subnets, why do they need a router to send traffic to each other?

If i have switch and i have 2 PCs connected to it. One of the PCs has an IP of 10.10.10.10 and the other has an IP of 10.10.20.10, why do they need a router to communicate? I thought devices connected to a switch sent/received traffic based on mac addresses so why would it matter if they are on different subnets?