Because of this, they have to have a single easily articulated point, and they have to beat the audience over the head with it.
Prior to this, I doubt whether directors, writers, or studios much cared if an unsophisticated viewer walked out of a movie with the "wrong" idea of what it "meant." The ability to attach multiple meanings, even multiple conflicting meanings, was seen as an inevitable aspect of art that should be embraced and engaged with. It was accepted that people would see a different movie depending on their background, their personal history, and their awareness of cinematic language. Supporting multiple readings was seen as a sign of depth and complexity, not necessarily a weakness.
Now the movies take a pragmatic, engineered approach to delivering a message. Ambiguity must be squashed. Viewer differences must be made irrelevant. The message takes precedence over art.
I think the interesting question is, why does the message now take precedence over everything else? What has changed? I see two possible answers.
First possibility, the audience demands a message. If the least-common-denominator viewer demands a message, and you are in the business of servicing that demand, you have to make sure you avoid any possible mishaps or misunderstandings in the delivery.
Second possibility, the makers of movies derive some personal satisfaction or social gain from broadcasting a message to the masses. They see the movies as propaganda rather than art. (Or perhaps a less active motivation: the makers of movies are afraid that there might be blowback from viewers attaching an unsavory meaning to a movie. They want to make sure that their movie doesn't become like Fight Club, a proudly embraced symbol of what it was meant to critique.)
Either of these would explain why movies are now engineered to deliver a single, unmistakable message at the expense of art and enjoyability. Or maybe there's another explanation. I'm just spitballing. I'd love to read more by somebody close enough to actually know what they're talking about.