https://cdnmetv.metv.com/Ykoxh-1467737260-466-lists-header.jpg

Such as (https://www.metv.com/lists/12-classic-hollywood-actors-who-appeared-on-the-love-boat) Olivia DeHavilland (https://www.goldderby.com/feature/the-love-boat-guess-31-oscar-winning-guest-stars-1202834805/), Mickey Rooney (https://www.classicfilmtvcafe.com/2015/12/love-boat-christmas-MeTV.html), and Lana Turner (https://oforinvolvingmotionpictures.wordpress.com/2015/02/14/stars-on-the-love-boat/).

Lana Turner (https://www.lana-turner.com/1985-love-boat/) on The Love Boat (https://webcache.googleusercontent.com/search?q=cache:X9QzMYdzB_UJ:https://www.chicagotribune.com/news/ct-xpm-1985-02-15-8501090621-story.html+&cd=21&hl=en&ct=clnk&gl=us):
https://qph.fs.quoracdn.net/main-qimg-6c56942addf1c6ed8267af4f318d8048

My guess is that they were persuaded by $$$.

It was an opportunity to work for people who'd long since passed their prime and get out there on a network show seen by millions.