- It's not as fast or inconspicuous as navigating to chrome://settings/passwords

- It does not present all passwords in a single list with the ability to show the one I'm interested in

- I would have to go to each site, allow Chrome to auto-fill, and then inspect the DOM and change the input type for each password I'm interested in. Far slower.

- It feels far more malicious to do what you suggest. Feeling is important. If I feel like I'm doing something bad, I'm less likely to do it. If I feel like I'm innocently navigating the Chrome settings page then I'm more likely to take a peek at your passwords.

- Far fewer people will be comfortable or familiar, or capable of using the DOM modification method. If a novice practices they are likely to get good at it. But that goes back to malicious intent.