Why do Kentuckians keep re-electing Mitch McConnell?
I think it is more accurate to say that both of them have transitioned into iconic status. The political/marketing machinations have lofted their identities so high that it barely matters that they are humans at all. They are brands. There is also the basic senate rules mentioned elsewhere, which institutionalize seniority.
Look at the list of longest serving US senators and ask yourself if Feinstein is really an anomaly:
https://en.wikipedia.org/wiki/List_of_members_of_the_United_...
Edit to add: remember when we used to have this conversation about Strom Thurmond or Robert Byrd? It's idiomatic.
But, when you look at Feinstein's accomplishments, she has represent her constituents as well as anybody... She authored the Respect for Marriage Act (undoing the conservative Defense of Marriage Act). She's pursued fair pay for federal wild land firefighters. She's protected millions of acres of federal land in CA for recreation. The list is extensive, as it should be for somebody of her tenure.
And with all that said, I do feel it's time for her to retire. I'm unconvinced on term limits, but I do dislike the tendency of long-time politicians to hang on well past their prime (and this applies equally on all sides of the aisle, and also to the courts).
The Republican Party of today is changing right before our eyes. Many hard-line issues are becoming soft - famously recently with Trump and his complete lack of religiousness.
Many of the other hard-line issues were distorted by political opponents, such as your claimed "gay" issue (when viewed through a religious lens, the marriage issue makes more sense, it wasn't really about people's sexual preference, it was about a specific word. if anything, republicans are absolutely terrible at getting their message across, consistently... but I digress...).
The point was, the younger generation of Republicans do not staunchly adhere to these "classical" Republican views - and the party is changing. The Republican Party seems to represent a lot more working-class people and minority groups today than a decade ago - voter segments that historically were under lock-and-key for Democrats.
To that end, the Democrat party is also changing; getting pulled a lot more left-ward than most average Liberals are comfortable with. It's a weird world where the likes of John Stewart and Bill Maher sound more like conservatives than liberals.
Both parties have found themselves within an identity crisis. My gut tells me there will be a course correction for the Democratic party not to distant in the future, and the Republican party will continue to "liberalize" as the younger generation takes over. We'll see where the road takes us all...
> Republican party will continue to "liberalize"