The way the voting and political systems work create a huge advantage for the incumbent, particularly in a state like California where there's a huge Democratic

The way the voting and political systems work create a huge advantage for the incumbent, particularly in a state like California where there's a huge Democratic majority. Primaries usually work on a plurality system, so even if the voters don't like the incumbent due to age, they have to all agree on the replacement, or the incumbent will win with less than 50%. Primaries also don't have a very high turnout rate, and so the incumbent can magnify their advantage in fundraising, political connections, etc. This goes double for a very old Senator that has had a long time to build up that political capital. There are also political costs to opposing the incumbent from within the same party. Running a competitive race will be seen as disloyal by many people in the party. It's much less risky to just wait out the incumbent and then run. For similar reasons, it's really suboptimal for the governor to be choosing Feinstein's interim replacement. Effectively one person will get to choose California's senator for the next 20-30 years.