A lot of them can't step down. They have debts that to repay that require them to stay in that position (both financial debts, and political ones), or family members who can only continue doing what they are doing if they stay in that position. Say a small family business that consistently gets works, but only to curry favor with the Senator. They quit, their grandchildren's business takes a dive, so the family begs them to stay on.

In Feinstein's case, its pretty clear she was forced to stay on by her party while they made arrangements for her successor, there is a lot of people and factions in California who want her seat in the Democratic party.

Also the competency crisis is real, alot of them dont have successors. Often by their own actions in an attempt to consolidate power, but they've left the field barren now thats its time for them to leave.