it is simple evolution has tended to make jaws stronger and bigger
a long arm is only a weak point against that .. because if you bite it and break it or tear off the loss of blood and bone pain is going to be much worse and could even cause death. against a small arm does not hurt so much for the least loss of blood and mass hosea and never cause death to be very easy to coagulate and weld, so that in case of withdrawal of a fight the animal would recover ... I doubt much that the irex or the spino would recover from a completely dislocated and torn arm, the infection would be terrible in comparison with their body and would make them lose a fight quickly. to the rex the only way to hurt him seriously is to bite his neck ... and it is huge and strong and the arms are much easier to hide from a bite than larger ones
Evolution strayed away from predators with short arms. Almost none of our predators today, have small arms. Maybe you could argue that crocodiles have small arms, but they also live in water. Not on land. And how can you say, that getting a small arm ripped off, doesn't "hurt" as much, as a bigger arm? You think it would hurt less, if you got your arm ripped off as a baby, than when you're an adult? And there's the same bloodvessels in a small arm, as there is in a large arm. You would lose the exact same amount of blood. And again, the exact same infections could occur. That doesn't have anything to do with the size of the arm.
You could also just rip open a T-Rex stomach, since it can't protect itself from anything attacking the body. It would be much harder for a Rex to get to a predator with bigger arms, because it can defend itself with both jaws and arms. Rex only has the jaws. It's like saying a boxer would have an advantage if he had small arms. It's the complete opposite. Boxers with long arms have the advantage.
Long arms could keep the Rex away and inflict damage at the same time. If the Rex falls down one time, it's over. If you have arms you can get up. There's not one thing better about having small weak arms as a predator.
The only reason evolution made Rex lose it's arms, was because it didn't need them. Because they were superior in size and didn't need to defend themselves...