Many Merrell styles use Vibram branded rubber soles. This material is specifically designed to be extremely durable. The ridges provide some added traction — particularly when ascending steep uphill climbs on the trail.
The compromise for making the rubber hardy and resistant to wear and tear is they can be a bit slippery on smooth/wet urban surfaces. They are designed to last on very rough off-road terrain, being a hiking shoe.
Over time and with gradual use, the sole will roughen up and provide better grip on urban surfaces.