Because Akainu is a soldier who follows orders, and he does so without questioning. Why would they be scared? We saw the Gorosei being in shock and awe hearing how Shanks and Whitebeard are meeting. We have the Gorosei discussing whether they should risk making Kaido angry by interfering with his fight.
Does that sound to you like 5 peak top tiers? Especially since those peak top tiers live on a huge continent with a vast amount of forces, including 3 admirals, a fleet admiral, couple of other top tiers (Sengoku, Garp), SSG, etc.