I find it helpful to imagine the plates are flexible and not rigid. They have forces acting on them that cause them to want to move in a direction, whether or n

I find it helpful to imagine the plates are flexible and not rigid. They have forces acting on them that cause them to want to move in a direction, whether or not they're capable of actual doing so. When they can't, pressure builds up until something gives and… you have an earthquake. This, plus plates deforming, I believe should solve your confusion. They are locked up against one another (in the case of strike-slip faults, moving past one another). There are also have other fault types, like subduction zones where one plate gets pushed beneath another (plate tectonics are 3D, not just 2D!). All this twisting and squeezing and pushing with nowhere to go until something fails. If this is a bad mental model, someone please correct me.