Croc teeth are designed to grab and hold—tear—but not sever. Believe what you want, but I’ve seen adult crocs, large adult crocs, being destroyed by a single adult hippo. You can’t get around the 1.5 ft tusks—and the hippo’s incredible agility. The croc doesn’t have anything to counter such a weapon. Hippo hide is 5 inches thick in some places and tough as leather—really, really thick leather. Essentially, it’s armor.
The crocs have flexible scales along their sides and underbelly—but they aren’t even close to armor. Crock teeth aren’t near 5 inches in length and the 5000lb bite force is distributed among all the teeth in contact—reducing it’s impact.
So, it’s stuck trying to rip skin that is designed by nature to be very tough to rip. The hippo also has a mouth width wide enough to encircle an adult croc, abdomen, where the croc can get it’s mouth around an adult hippos extremity but forget a back-breaking grip. If the Hippo will sit still, like a dead one, the crocs can eventually tear it apart—especially after decomposition has weakened the tissue strength.
The reason crocs live together with hippos is because hippos don’t eat meat and crocs usually understand it’s foolhardy to attempt to attack an adult hippo. So, they don’t even try. No interest on the hippos side and nearly assured destruction on the croc’s side.
Hippos tend to not normally bother crocodiles, either.
Crocs sometimes do get in trouble from time to time trying to get a calf. Usually, it’s a seriously bad move on the croc’s part. Note the difference in tusk length and tooth length in the pictures below.
Pictures borrowed from the internet