in theory the devs and product managers could test in staging, but in practice that's impossible to enforce; for one, some things are just very hard/impossible to test in staging; additionally, when the "testing" looks exactly like legitimate traffic it's really hard to enforce that in any way, devs and product managers are going to reach for the easiest tool, and that's going to the website to see if the change they just deployed works as expected (or to try an reproduce a bug a user just reported)