Your grip fails before your legs can finish the set. Use wrist straps looped under the dumbbell to offload the weight from your hands, or switch to a mixed grip on the barbell for better security without straps.
If your hands always give out before your legs do, then here's 2 things I want you to try. Number one, get wrist straps. These will go around your wrist, and then you can use this to go under the dumbbell, and this will basically hold the weight for you and you'll be able to do much heavier. And the second hack, if you don't have wrist straps yet, but you do have access to a barbell, then you can put one hand over and then one underhand. And this kind of grip will help you lift heavier weight without having your hands be the thing that holds you back.
