Well, #19, the beauty of using a computer instead of, say, a notepad and pen is that our keyboards have all these useful little buttons that we can use to go back and correct mistakes without making it look messy.
Guess what? People make typos all the time. It doesn’t mean that they are stupid. If you’re typing quickly mistakes do happen.
As for “yeah but you can proofread”, when you go back and reread what you wrote, you tend to read it how you intended it, therefore missing the mistakes you made. That’s why when people write papers they have other people edit their papers.