Your comparison is flawed.
Models are usually independent contractors who are signed with an agency or various agencies and thus are not subject to employment law. The agencies are there to put out their name and portfolio to attract gigs.
TV personalities are employees of the station and are subject to employment laws which included anti-discrimination.
It's pretty simple if you want to fire someone you better have a valid reason (history of poor performance, violating polices & procedures, etc.). If that employee has a history of great performance and reviews, but happens to be 60 years old and you fire them without a reason then you are going to get sued for ageism. It's actually a pretty simple concept.