Why doesn't instagram login with javascript work?

Question

JavaScript

Go to the login page:
https://www.instagram.com/accounts/login/
Run:

document.getElementsByTagName('input')[0].value = 'login';
document.getElementsByTagName('input')[1].value = 'password';

You can check your password for every fireman:

document.getElementsByTagName('input')[1].type = "text";

Press "Login":

document.getElementsByTagName('button')[0].click();

Outcome:

The username you entered does not belong to the account. Check your username and try again.

I'm trying to understand how Instagram recognizes scripted fields?
The chrome inspector suggests that the Values ​​in the text fields remain empty , although the script explicitly fills them (and this is visible on the screen).