JavaScript is a requirement on most forms due to either designer or client expectation. They want styled, custom client side validation. They want a visual loading state while the form is being fetch'd to the server/service. They want a nice success message to display inline once the submission is complete. Rarely is it custom controls or weird editing flows in my experience - it's much simpler than that.

And we do it because they're paying the bills - there are more important hills to die on.