One reason we might never Sentry in the MCU is the simple fact that he’s so overpowered that he could beat just about anyone that isn’t cosmic-based, and even then there’s no telling where his upper limits would be set. This is assuming that the MCU would leave his power levels as they are and not overplay any weaknesses he might have, but that’s kind of hard to gauge since thus far the MCU has lowered the power levels on a lot of characters and ramped up others to a level that they never experienced in the comics. But the Sentry is the type of character that can throw hands with the Hulk, and has, and can take on some of the most powerful beings in existence like it’s a day in the park. In other words, the guy might not work as a character simply due to the fact that he might need some crippling weakness that would make little to no sense. It would be great to see him show up, but at the same time it’s something that a lot of people are thinking might not happen.
There’s not much of a way to know at the moment since there are a few things that the MCU are keeping close to the vest, and it’s possible that they might be thinking of some way to introduce this character without making him a type of Superman character that basically does anything with little to no effort. While this does invite bigger and more impressive fights and struggles in the MCU that could keep expanding it in a cosmic sense, it could also come off as a kind of lame attempt to copy a DC character, as there have been plenty of accusations handed out over the years when it comes to the similarities between several of the characters that each company pushes. Of course, if one is going to be entirely honest, there’s been more than one Superman comparison in the Marvel universe since Sentry is one, and characters such as Vulcan, Gladiator, and Hyperion have also been likened to the man of steel.
If Sentry ever does make it into the MCU, and that’s a big if, at this time, it could happen when the cosmic threats become even more apparent, and perhaps when the earth is on the verge of being annihilated. In a way, Sentry is like a weapon that doesn’t need to be handled until the situation is really that intense. This means that he’s someone that can’t be controlled as easily and needs to be handled with caution, especially if he were to lose control and unleash the Void, the other half of his personality that can be just as destructive but doesn’t care if it is. A lot of heroes have dark sides to them, but Sentry’s is more than a little dangerous since the Void is the kind of thing that could very easily cause the type of damage that would be catastrophic and likely world-changing. The Void is the other half of Sentry, his darker half, and it bonded to him when he took the serum that gave him the powers he now has.
There could be a number of different reasons that Sentry hasn’t been mentioned yet, but it does feel as though fans are going to be wondering where he is at some point since despite being forgotten now and then, Sentry is still a very important asset that can be called upon when needed. He was a deciding factor in World War Hulk, which is another idea that a lot of fans might like to see eventually since it was another moment in Marvel history that could easily be adapted and turned into something that might keep the fans happy. As an inclusive storyline, World War Hulk would be something that a lot of fans would likely get behind without question since it brought in everyone from the Avengers to Ghost Rider and managed to be one of the more enticing stories that Marvel still has to offer. Thanks to the buildup to Infinity War, any chance of World War Hulk might have been put on hold or lost since Thor: Ragnarok was about as close as it came since now Bruce Banner and the Hulk are merged and Mark Ruffalo’s character actually despises violence.
If the Sentry ever does make it into the MCU it almost feels as though he’ll be a supporting character or a glorified extra, or possibly just a mention that doesn’t go anywhere and becomes kind of a joke. It’s understandable that Disney is in charge and that they want to use the characters that they believe are more popular, but at this point, adding in the Sentry like a great idea.