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While there are volcanoes that are brought about by plate tectonics (like the Cascades in Washington and Oregon), there are also volcanoes, that while affected by it, are not caused by plate tectonics (the Hawaiian volcanoes). It is believed that in the Hawaiian volcano chain, a stationary hot spot is poking through the lithosphere, creating a volcanoes as the lithosphere passes over it. This is why all of the Hawaiian islands are in a line. The plates move over a stationary hot spot. If the plates were not moving, then the hot spot would deposit all of its extrusions in one area, making a monumental volcano.

This is what appears to have occurred on Mars. If there was a previous volcano chain when the plates actually moved, it is likely that it has been destroyed by the monumental scale of the Tharsis volcanoes. Now that the plates are stationary, the volcanoes can only grow upwards and outwards. What limits the size of the volcanoes isn't the speed of the plates (like on Earth), but instead, it is time.

Imagine if the plates on the Earth did not move. How big would be the one Hawaiian super-volcano be then? It should also be noted that if the plates did not move, then they wouldn't be recycled every 250 million years--who knows how stable the hot spot under the Hawaiian islands would be when measured over billions of years. Certainly the Hawaiian plume has turned on an off (since if it didn't we'd probably see the Hawaiian islands lead all the way to Australia).