The cakes have to be baked in pans with sharp corners, filled dead level and cut flat with a knife.
You have to apply a good 3/8 inch of buttercream because smoothing takes off a significant amount. This will depend on the exact buttercream recipe you used.
You might find better success by using ganache instead of buttercream, because ganache is a more constant composition and more amenable to being cut flat with a heated metal bench scraper (dipped into boiling water). Or you could make chocolate panels to apply for that final flat finish.
FYI the cake in the photo is fondant...don't feel bad.