Disclaimer: I am currently in the process of trying to verify if the Stark Gauntlet was made out of Vibranium. So far the only theory I can think of for why it might be is that he could have re-purposed Vibranium from Ultron or someone could have taken some out of Wakanda in the time gap.
With that said I would take this answer with a pinch of salt.
The Infinity Gauntlet is made out of Uru, the same as Mjolnir, and Stark’s Gauntlet is made out of Vibranium, the same as Cap’s shield. We’ve seen these two metals come to blows with each other when Thor has hit Cap’s shield with Mjolnir over the years and it as much as dented it so who’s to say it is stronger? In fact Vibranium is the strongest metal on Earth.
Bruce Banner: I don't follow. What comes out of Wakanda? [looking at Steve's shield]
Tony Stark: The strongest metal on Earth.
Avengers: Age of Ultron
Add onto that, that the enchanted Uru is essentially just advanced science that people don’t understand and so call magic.
Thor: Look your ancestors called it magic. You call it science. I come from a place where they're one and the same thing.
Lastly, note those who made the Stark Gauntlet: Tony, Banner and Rocket. Tony and Banner have had exposure studying Infinity Stones in the past and Rocket is an extremely skilled engineer. There’s also the possibility that they took the original Infinity Gauntlet from Thanos at the beginning of the film and so have improved upon the original design.