The main reason that GGG didn't walk right through Jacobs and knock him out in round 4 or 6 was obvious and one that I never saw mentioned on any boxing forums or in the media prior to and after their fight.
Look at GGG in interviews in the week before the fight, it was obvious he had a serious herpes simplex ("cold sores") outbreak on his upper and lower lips. If he is like most people, a cold sore is a result of weak immune system and will seriously affect strength, stamina and mood negatively even though this can be subtle in some people. Even if the cold sores had mostly cleared up on fight night, in his last week before the fight GG probably was running a slight fever, had reduced energy and felt like ****, some people get back pain, headaches are common etc. This is most likely why GGG had a "bad" night against Danny Jacobs.
and another point that everyone talks about is GGG's power...most say he was simply born with "heavy hands" or he uses special training techniques (which is partly true). Have a look at GGG's forearms, they look like they belong on a cruiserweight boxer who lays bricks during the day. His forearms are big and heavy and along with focus, timing, solid footwork and body movement etc , this is where the power comes from, that's another point that I haven't read or heard about.