Because the internet is a black hole; it's impossible to tell what's a "lie" and what's a "leak." Even random people reporting online sometimes report "leaks." If claims about Marvel projects are "leaks," Marvel either confirms them at events with its journalists or fans. Or, when Marvel's film and series actors can't reach an agreement with Marvel on a project, they announce it themselves. Anyway, back to the topic, because there are plenty of people attending these events online and posting on Twitter (and there are also plenty of people on Reddit who went to test screenings and wrote the entire movie, down to the After Creeds scene, and their writing turned out to be true). There are even plenty of people who believe the news on sites like Comic Books. Furthermore, "people" tend to believe any news online unless it's political news.