It's time to face the facts: Ant-Man is nowhere to be seen in Avengers: Infinity War, and it's not because he spends the entire film in the subatomic size. Why Ant-Man isn't in Infinity War is actually explained by a pretty good reason; the hero is missing out on the first big showdown against Thanos likely because of the ankle monitor slapped on his leg in the trailers for Ant-Man and the Wasp.
Infinity War confirms what fans have suspected since the release of the Ant-Man and the Wasp trailer in January: Scott Lang is still suffering the consequences of his actions in Captain America: Civil War. At the end of Civil War, Ant-Man was imprisoned on the Raft along with the rest of Team Cap for going against the Sokovia Accords, only to be broken out by Cap himself. However, instead of following Captain America's lead and living life as a fugitive, Ant-Man and Hawkeye have taken a different route. As revealed in Infinity War, both superhero dads have instead made deals with the government, allowing them to return home safely to their families as long as they don't engage in any unsanctioned superhero activity. Hence their absence from Infinity War.
It's likely fans of Scott will get to see exactly what he's been up to since Civil War in Ant-Man and the Wasp, the Ant-Man sequel hitting theaters just a few months after Infinity War, but it's unclear exactly when the film takes place in the Marvel Cinematic Universe timeline. Following normal MCU guidelines, Ant-Man and the Wasp's release date would mean it takes place after Infinity War, but the sunny and normal world shown in the early trailer for the film doesn't really jive with the whole "end of the world" vibe of the new Avengers film, which has made some fans think that the movie actually takes place before Thanos invades Earth.
Infinity War directors Joe and Anthony Russo assured reporters during a set visit that Ant-Man and the Wasp would tie into Infinity War in some capacity, though they refused to give out any detail. "As far as plot elements that actually drive the toward the story, again, I think Marvel does a great job of segmenting the movies so that you can have your own experience in each film," Joe said, via IGN. "I think from a plot standpoint, if there's any corollary, Ant-Man and the Wasp probably has some elements that stitch in."
The official synopsis for the film says that it will pick up "in the aftermath of Captain America: Civil War," so it seems most likely that the sequel will focus on what happens to Ant-Man directly following his release from prison and before Thanos emerges in Infinity War. However, based on what the Russos revealed, it's likely that the movie will also include some references to Infinity War, so best guess is that it straddles both before and during the events of the Avengers movie. It will be interesting to see how the film addresses Scott's decision not to break his parole and join the fight considering the fact that he's never really cared about breaking the law before, but it's obvious that Marvel wanted to give Ant-Man and the Wasp both a chance to grow as heroes before getting them involved in the Avengers massive fight.
Allowing Ant-Man and the Wasp to happen before dragging Ant-Man back into the Avengers fold makes sense, not just character wise (Scott loves his daughter, so making a deal with the government is a no-brainer), but also on a strategic level for Marvel Studios. By giving the Wasp a chance to establish herself as a hero in her own right, it will allow them to get another woman on the Avengers team in Avengers 4, and let's face it, the group could some more ladies on-board.