Obviously, I'm very late to this topic. I haven't seen all Marvel films, but I have seen a majority - enough to have an informed opinion on the matter.
But, yes, they are. Seriously, though, it's a matter of semantics. Movies are entirely subjective. A lot of people have mistaken Martin Scorsese's comments on MCU films being "not cinema" as a value judgment. But it isn't really, he was just trying to distinguish the type of film he likes to watch, and Marvel. Granted, some others, such as Coppola have come out and made pretty blatant value judgments. Either way, the important thing to note here is, the value of cinema is in the eye of the beholder. Indeed, what the word "cinema" means is itself very much a personal take.
There are many great aspects to Marvel CU films. They bring in fresh new directors off the independent scene, and give them nearly unlimited resources. While the films are pretty formulaic, and need to be consistent to the universe, each film definitely has its own flair brought by the fledgling director and their crew. It is also an interesting experiment to build a vast, overarching story over a dozen or more films. Yeah, I know, that sounds a little bit like an episodic TV series, and fair enough, that's kinda true. However, where Marvel diverges from the formula by offering much more creative freedom to each episode's director, and lavishing them with massive budgets.
Of course, the massive budgets means the Marvel films bring significant technical innovations each time. Whether it be camera, rigging or sound technology, special effects, or obviously, visual effects & CGI - each Marvel film pushes filmmaking tech forward for the entire industry.
Unfortunately, the films are pure formula. The model of working with a different director, often relatively inexperienced indie directors, does keep certain things fresh and interesting, but the overarching scripts are all too familiar. Some films do begin with fairly interesting premises, but pretty much all devolve down to a fight against the "conflicted" villian who thinks they are actually saving the world.
But that's not a criteria for a film to be "cinema" or not. To me, any work that uses the audiovisual medium, in any way, is cinema. Whether it's good cinema or bad cinema - is an entirely different argument. This goes beyond Marvel, of course. I don't think a certain type of film is inherently of greater value than another. It just depends on how good the film is by itself. All that said, I would say most Marvel films are mediocre cinema, while a handful are fairly good. At the very least, all of these films are better than some of the awful blockbusters of the 90s.
Perhaps the most interesting thing about the MCU are the Avengers films. No other film in history has the privilege of pulling together so much development from the other films, all towards one film, and across so wide an audience. I believe the directors of the latest Avengers movies call it "emotional capital" - and that's a pretty apt description. Whether one likes the films or not, one has to appreciate ideas like these as cinematic innovation.