Tony Stark regularly updates his Iron Man armor after his previous battles, but for Avengers: Endgame, he did not create an armor against Thanos.
Worst nightmare of Tony Stark (Robert Downey Jr.) came to life in Avengers: Infinity War when Thanos (Josh brolin) finally made his presence known. After the 2012 Battle of New York, where he nearly died transporting a nuclear bomb into space, Iron Man had been hunted by an unnamed threat. He became obsessed with making sure Earth was ready for another alien attack. Which resulted in some questionable decisions, like creating an army of robots like Ultron.
Tony Stark, known for regularly updating his creations, wore a number of large Iron Man suits on his way through the Marvel Cinematic Universe, each better than its predecessor. In addition to creating things for himself, he also built armor and weapons for other people, especially those closest to him. He was behind the War Machine armor of James Rhodes (Don Cheadle), two of Spider-Man's suits from Peter Parker (Tom Holland) and of Pepper Potts (Gwyneth Paltrow). He also played with the Avengers gadgets to make sure they were always ready for battle.
In addition to simply wanting his allies to have the latest and greatest technology, Tony's updates were always motivated by his past experiences. For example, he included a parachute in the first Stark suit of Spider-man after Rhodey's plane crash in Captain America: Civil War and made Bleeding Edge Armor operational in space for Infinity War.
Given this, fans would have assumed that after the Avengers' defeat to the Mad Titan in Infinity War, Tony would be inclined to create armor against Thanos or a similar powerhouse.
Like the suit Hulkbuster which debuted in Avengers: Age of Ultron (2015) and reappeared in Infinity War, the Thanosbuster would be a suit specifically designed to fight the villain. Given that Iron Man personally fought his nemesis, it would have been quite easy for him to come up with designs that could better prepare him for a possible later encounter. However, that was not the case, why?
Tony didn't make a Thanosbuster simply because he didn't think it made sense to create one. Unlike the other heroes, he had no hope of undoing the "snap". The Mad Titan won and for him, there was no way to get back at the villain. While the rest of the remaining heroes obviously didn't feel this way when interrogated for any information that might help him locate the Mad Titan, Iron Man was simply defeated. This is why he felt reviled when Steve Rogers / Captain America (Chris Evans) began asking him questions that forced him to relive the nightmare of losing to Thanos on an alien planet, followed by being stranded in space for three weeks, which almost killed him.
He didn't prepare like he used to do before.
Iron Man was convinced that there is no way to undo the snap. He told Scott Lang / Ant-Man (Paul Rudd), Natasha Romanoff / Black Widow (Scarlett Johansson) and Steve when they sought his help for the temporary robbery. It was only after that meeting, followed by seeing her image alongside Peter Parker that she had an epiphany. Since they were also mission-focused and did not expect to meet Thanos again, creating Thanosbuster armor was simply not a priority for him at the time. So he focused on creating something that could hold the Infinity Stones.
But he did not think of all the variables. Since the most sensible thing is that if you do a robbery in time you must be prepared for everything, that includes creating a tight armor to defeat Thanos or someone similar. But his firepower was totally defeated again against the Thanos of 2014. So it shows how ill-prepared Iron Man came to the final battle.
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