Even in the age of NDAs, closed sets and penalties for any actor who talks too much, secrecy is a difficult thing to come by in Hollywood, particularly around tentpole releases that could well make billions of dollars. After all, anything that pulls in that much money for studios at the box office could potentially bring in a big pot for film websites looking for scoops too. So it's no wonder they dig a little harder.
Marvel Studios know all too well the dangers of too much information leaking. They've seen the Giant Man reveal from Civil War revealed early, most of the key plot-points of Age Of Ultron hit the web before the film's release and Spider-Man: Homecoming's twists all laid out prematurely. And that's just a drop in the ocean.
Unfortunately for them, they have actors (like Mark Ruffalo) who occasionally spill the beans and work with toy companies (like LEGO) who include plot points in their tie-in sets. And there's still the age-old concern of set invasions to deal with too. They're getting smarter with it - like by apparently shooting consciously misleading scenes in Atlanta to confuse anyone looking too closely - but there's still a lot of information about Infinity War out there that they'd probably prefer wasn't.