She was good looking at that time. And this way Arnold also married into the Kennedy family.
He was very ambitious and wanted to be more than just a bodybuilder and actor.
He succeeded.
Yes, very shrewd. Marry a decent looking woman from a wealthy powerful family to ensure your children will be well-off in life and fuck other women on the side. If Arnold was banging the unattractive maid, I'm sure he was fucking anything that crossed his path.
Ironically, the maid took a page from his playbook - getting pregnant by Schwarzenegger ensures that her son will also be well off in life.