The fan theory, the Marvel icon isn’t playing a series of unrelated characters in his many cameos, but is actually the MCU’s version of The Watcher. A character named Uatu, The Watcher is part of an alien race whose job it is to keep an eye on significant events throughout history but never intervene. That would explain why Lee always appears whenever a Marvel hero leaps into action to save the world, as such superheroics would certainly be worthy of record by The Watcher.
Of course, the chances that the MCU ever acknowledges or even intends for Lee to play The Watcher are probably pretty slim. After all, the boring answer here is that Lee is involved because of his recognizability by fans and his integral role in co-creating some of Marvel’s most iconic characters back in the 1960s. His consistent appearances in the films are little more than a show of respect to Marvel’s roots, and Lee has, in many ways, become one of the company’s biggest spokespeople. Even some non-comic readers know to look for him in every MCU film.