Here's why Tony Stark/Iron Man (Robert Downey Jr.) didn't build a Thanosbuster armor in Avengers: Endgame. The genius, billionaire's worst nightmare came to life in Avengers: Infinity War when Thanos (Josh Brolin) finally made his presence known. After 2012's The Battle of New York where he almost died hauling a nuclear bomb off to space, Tony had been haunted by an unnamed threat. He became obsessed with making sure that Earth would be ready for another extraterrestrial attack which resulted in some questionable decisions such as creating an army of robots in Iron Man 3 and Ultron in Avengers: Age of Ultron.
Known for regularly updating his creations, Tony wore a string of great Iron Man suits in his stint in the MCU — each one better than its predecessor. Aside from creating things for himself, he also built armors and weaponry for other people, including those closest to him. He was behind James Rhodes' (Don Cheadle) War Machine armor, two of Peter Parker's (Tom Holland) Spider-Man suits, and Pepper Potts' (Gwyneth Paltrow) costume. He also tinkered with the Avengers' gadgets to make sure that they were always battle-ready. Aside from simply wanting his allies to have the latest and most powerful tech, Tony's upgrades were also always motivated by his previous experiences. For instance, he included a parachute in Spider-Man's first Stark suit following Rhodey's flying accident in Captain America: Civil War and made the Bleeding Edge Armor operational in space for Infinity War.
Given this, fans would've assumed that in the aftermath of the Avengers' loss to the Mad Titan in Infinity War, Tony would be inclined to create a Thanosbuster armor. Similar to the Hulkbuster suit that debuted in Age of Ultron and made a re-appearance in Infinity War, the Thanosbuster would be a suit specifically designed to battle the purple villain. Since Iron Man fought his nemesis personally, it would've been fairly easy for him to come up with designs that could better prepare him a potential later encounter. However, that wasn't the case — why?
Tony didn't make a Thanosbuster simply because he didn't think there was any point in creating one. Unlike the other heroes, he had no hope of undoing the Snap. The Mad Titan won and for him, there was no way for them to get back at the villain. While the rest of the remaining heroes obviously didn't feel this way as they grilled him for any information that could help him locate the Mad Titan, Iron Man was simply defeated. This is the reason why he was repulsed when Steve Rogers/Captain America (Chris Evans) started asking him questions forcing him to spotting his and Peter's upside-down image that he had an epiphany. Since they were also focused on the mission and didn't expect to re-encounter Thanos again, creating a Thanosbuster armor just wasn't a priority for him at that point.
Tony being the first core Avenger to give up on the idea that there's a way to get back at Thanos was understandable. For the longest time, he single-handedly dealt with the burden of knowing that the Mad Titan would attack Earth again (or a threat like him). Iron Man suggested that the Avengers be more prepared for the Mad Titan's imminent attack, but it fell on deaf ears — at some points, he wasn't just ignored, but also antagonized. Even when the Avengers broke up after Civil War, he worked on his own, updating his suits, and making sure he would be as ready as he could possibly be for when Thanos struck. And the fact that he moved on with his life and was happy with his family following the Snap in Avengers: Endgame after abandoning all hope makes his death so much more depressing.
