Let's say you're set up on a blind date, and the guy shows up 10 minutes late in a ratty t-shirt. He says your name wrong, and you notice he smells terrible. Now, while you might later discover that he's smart and funny and shares all your common interests, it's going to be hard for him to win you over after such a bad beginning. Plus, you have to wonder how serious the guy is about the date if he wasn't willing to take a shower and put on a clean shirt. Sitting down to read a script is essentially a blind date.
All writers make mistakes, and typos can be hard to catch.
Often, I'll be on my third revision of a script and suddenly notice a glitch that must have been there from the start, yet eluded both myself and my trusty assistant. But whenever I find one of those, I feel like an idiot. I don't like to feel like an idiot. That's why I proofread so carefully. I'm a high school student and aspiring screenwriter.