It's just a random chance of happening, kind of like a shiny, but less rare. If something had a 50% chance of appearing it could technically not appear for thousands of encounters. Your just getting very unlikely. The way to get the Smeargle is:
Tap on your pet in the bottom left corner of the main screen if Pokemon Go and hit Play!
Then take pictures with your pet, as many as you want. Each picture can have a Smeargle in it so you should take plenty.
Then when your done taking pictures, on the top right of your screen there should be a button that has an arrow and a cell phone on it. Tap that and it will let you scroll through all your photos. If a Smeargle appears hit the x on the bottom to exit out of the photo reel and you should be forced into a "battle" with a Smeargle.
Hope I helped Ill add this to the answer