I did so for good reason: that's what the screenwriting books told me to do. There's been a long-time emphasis on structure, and in particular on a very tightly structured three-act paradigm. This concept was voraciously sold for years by the late Syd Field. It has its merits and is actually a pretty important thing for young writers to learn.
The problem is that the various forms of the three-act structure have essentially become screenwriting rules in Hollywood—rules that are assumed to never be broken. When you show a script to many industry people, they will point to what they deem as structural discrepancies (i.e. this plot point doesn't happen on the page it should) as if that's what is of prime importance.
There are a couple reasons why this overemphasis on structure has become so ingrained in our screenwriting culture. Movies are expensive to produce, and therefore studios want to maximize each film's potential audience. A bad risk or two could sink a studio. Readers are taught to look for what has worked in the past—for broad audiences. Structured paradigms fit this bill.
Added to this is the shine of the entertainment industry that lures many people to it, many who dream big, of becoming successful screenwriters and of receiving all the glory that naturally comes with. The reality, however, is that most people who dive into the screenwriting realm aren't natural writers, never will be, and are too easily drawn to whatever technical advice that they feel will improve their craft.
The quantification of the art of screenwriting does help produce the dozens of mediocre movies we see every year. It helps the large studio system stay solvent. It isn't the cornerstone of quality writing, however—not by a long shot.
I’m glad I wrote so many sub-par scripts while having the wrong blinders on, because you have to get so many hours in (regardless of your craft) to improve to the point where you're actually pretty good at what you're doing. It wasn't until I got a good chunk of the way through the MFA in Creative Writing program that I attended until the light bulb went off in my head.
Sure, structure is important. It's not what separates the best written narratives from the rest, however. That comes down to something else. My focus was misplaced. It should have been on what matters most: the characters.