Why did it take over a billion years for living organisms to appear after the earth was formed?

Current estimates are that life emerged on Earth between 3.5 and 3.8 billion years ago, but it may have emerged even earlier than that.

Life, according to our best hypotheses, emerged almost as soon as the conditions were favourable. Before that, the planet's surface was simply too hot, its atmosphere and surface conditions too corrosive. After all, the planet was still cooling down and outgassing following its coalescence from interplanetary debris.