Why Capybara requires button for submitting forms
Despite being a super common request, Capybara's API doesn't give you a way to submit forms directly (without hitting a submit button). The denial to do so is actually a principled stance, as you can read for yourself in this pull request. In a nutshell, Jonas believes its a bad practice to do so, plus there is no standard way to support the functionality across all browsers.
Workarounds exist, but seem clunky.