You DO get it
Each and every one of you. No matter how ashamed you are of having a brain, no matter how afraid you are of admitting you're capable of independent thought.
All the "biggest trash ever/please explain" posts already contain the answers they're purportedly seeking.
EVERYONE got that the married couple has issues. Well, THAT'S the point, not why they have it. If you're interested why, go read an essay on marital problems.
EVERYONE got that Borgman is destroying a perfect suburban upper middle class life. THAT'S the point, not why. Do you REALLY need explained why a hobo would want to destroy a life of a rich guy who almost beat him to death?
EVERYONE got that the children are being abused under mothers very nose, and she completely ignored it. THAT'S the point, not why and how.
What would the film gain if the whys and hows were explained? Virtually nothing. It's themes would remain the same, it's points also.
If you're interested in why and how, go read a book on science, or a business report, or a manual for a dishwasher. That's about why and how. Art is not. And neither is entertainment.