Writing automated tests is not the same thing as testing it. Senior engineers write a lot of code, write some tests, do some manual validation. But its usually far from complete.

Other roles in a software team can handle making a complete test suite for a project, and making sure that it covers all of the required end user functionality. In some shops they will push all of these together into a "fullstack" kind of role, in other software shops they will have dedicated team members who's job it is to handle some of this burden. Interns historically filled in the glue for this.