Patches of grass are typically not RIGHT NEXT TO Pokemon Centers which tend to be located in towns, whereas patches of grass are usually located OUTSIDE of town.
Besides, on the trip back to the Pokemon Center, you can be attacked by OTHER wild Pokemon, thus delaying the journey back to the Center... or in some cases it may be more prudent to push onward and complete the route you're on, because it would be faster to complete the route and go on to the NEXT Pokemon Center rather than return to the one you were just at.