As a film maker who studied film, the reason why so many movies are so bad are manyfold:

  - making movies is hard. A lot of things that require years to master need to go right. A *ton* of tech is involved. 
  
  - making movies is expensive. Money alone won't make you a good movie, but many productions are so on the edge that some choice they had to make for monetary reason will cause the bad. 
  
  - making movies is complex, that means making a masterful one requires multiple botched attempts and experiences by all people involved. These botched attempts are also what you see.  
  

I can't stress enough how hard making a movie is, even in comparison to complicated tech problems, programming etc.