Simply they don't bother taking down the advertisement once hired.

It's the lowest priority of job for the hiring team and the role is normally forgotten about once the hire has been onboarded.

I worked for an animation studio and they didn't take down roles.

Companies don't take down listings for many reasons: make it look like they have more money to expand than they actually have, slow down the job seeker so their competition moves just slightly slower, there's no benefit in taking it down, if someone is actually reading the applications maybe an exceptional applicant will come that fits for another role in the company, drive up costs for the competition (hey boss everyone is hiring for my role so give me a raise), etc. [edit: gram mer and spolling]