> LastPass fills out my username and password on modals just fine. Tested it out on Hertz just now. If other password managers don't... then they should be improved, no?

Yes, it works if you visit the site, bring up the login model, and then tell the password manager to fill it in (or are configured to auto populate).

However, many password managers have a feature where you find the site in the password manager, and click to visit it, at which point it loads the site and automatically logs you in. When you get used to it, it is a convenient workflow basically merging bookmarks and passwords management into one tool. This is the feature that modals break.