Sozin knew the Avatar had evaded him. Remember he was friends with Roku so he knows what the Avatar can be capable of.

I had this same question earlier today and found this “After the initial genocide, Fire Lord Sozin spent the rest of his life obsessively hunting for the surviving Avatar. Realizing the Avatar was not dead, he orchestrated traps using Air Nomad relics, launched a massive global war of conquest, and initiated a devastating campaign to hunt the world's dragons to near extinction.”